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  <title>C++ Object Persistence with ODB</title>

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  <div id="titlepage">
    <div class="title">C++ Object Persistence with ODB</div>

    <p>Copyright &#169; 2009-2025 Code Synthesis</p>

    <p>Permission is granted to copy, distribute and/or modify this
    document under the terms of the
    <a href="http://www.codesynthesis.com/licenses/fdl-1.3.txt">GNU Free
    Documentation License, version 1.3</a>; with no Invariant Sections,
    no Front-Cover Texts and no Back-Cover Texts.</p>

    <!-- REMEMBER TO CHANGE VERSIONS IN THE META TAGS ABOVE! -->
    <p id="revision">Revision 2.6, March 2025</p>
    <p>This revision of the manual describes ODB 2.6.0 and is available
    in the following formats:
    <a href="http://www.codesynthesis.com/products/odb/doc/manual.xhtml">XHTML</a>,
    <a href="http://www.codesynthesis.com/products/odb/doc/odb-manual.pdf">PDF</a>, and
    <a href="http://www.codesynthesis.com/products/odb/doc/odb-manual.ps">PostScript</a>.</p>
  </div>

  <hr class="page-break"/>
  <h1>Table of Contents</h1>

  <table class="toc">
    <tr>
      <th></th><td><a href="#0">Preface</a>
        <table class="toc">
          <tr><th></th><td><a href="#0.1">About This Document</a></td></tr>
	  <tr><th></th><td><a href="#0.2">More Information</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th colspan="2"><a href="#I">PART I OBJECT-RELATIONAL MAPPING</a></th>
    </tr>

    <tr>
      <th>1</th><td><a href="#1">Introduction</a>
        <table class="toc">
          <tr><th>1.1</th><td><a href="#1.1">Architecture and Workflow</a></td></tr>
	  <tr><th>1.2</th><td><a href="#1.2">Benefits</a></td></tr>
	  <tr><th>1.3</th><td><a href="#1.3">Supported C++ Standards</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>2</th><td><a href="#2">Hello World Example</a>
        <table class="toc">
          <tr><th>2.1</th><td><a href="#2.1">Declaring Persistent Classes</a></td></tr>
          <tr><th>2.2</th><td><a href="#2.2">Generating Database Support Code</a></td></tr>
          <tr><th>2.3</th><td><a href="#2.3">Compiling and Running</a></td></tr>
          <tr><th>2.4</th><td><a href="#2.4">Making Objects Persistent</a></td></tr>
          <tr><th>2.5</th><td><a href="#2.5">Querying the Database for Objects</a></td></tr>
          <tr><th>2.6</th><td><a href="#2.6">Updating Persistent Objects</a></td></tr>
	  <tr><th>2.7</th><td><a href="#2.7">Defining and Using Views</a></td></tr>
          <tr><th>2.8</th><td><a href="#2.8">Deleting Persistent Objects</a></td></tr>
	  <tr><th>2.9</th><td><a href="#2.9">Changing Persistent Classes</a></td></tr>
	  <tr><th>2.10</th><td><a href="#2.10">Accessing Multiple Databases</a></td></tr>
          <tr><th>2.11</th><td><a href="#2.11">Summary</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>3</th><td><a href="#3">Working with Persistent Objects</a>
        <table class="toc">
          <tr><th>3.1</th><td><a href="#3.1">Concepts and Terminology</a></td></tr>
	  <tr><th>3.2</th><td><a href="#3.2">Declaring Persistent Objects and Values</a></td></tr>
	  <tr><th>3.3</th><td><a href="#3.3">Object and View Pointers</a></td></tr>
          <tr><th>3.4</th><td><a href="#3.4">Database</a></td></tr>
          <tr><th>3.5</th><td><a href="#3.5">Transactions</a></td></tr>
	  <tr><th>3.6</th><td><a href="#3.6">Connections</a></td></tr>
	  <tr><th>3.7</th><td><a href="#3.7">Error Handling and Recovery</a></td></tr>
          <tr><th>3.8</th><td><a href="#3.8">Making Objects Persistent</a></td></tr>
          <tr><th>3.9</th><td><a href="#3.9">Loading Persistent Objects</a></td></tr>
          <tr><th>3.10</th><td><a href="#3.10">Updating Persistent Objects</a></td></tr>
          <tr><th>3.11</th><td><a href="#3.11">Deleting Persistent Objects</a></td></tr>
	  <tr><th>3.12</th><td><a href="#3.12">Executing Native SQL Statements</a></td></tr>
	  <tr><th>3.13</th><td><a href="#3.13">Tracing SQL Statement Execution</a></td></tr>
          <tr><th>3.14</th><td><a href="#3.14">ODB Exceptions</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>4</th><td><a href="#4">Querying the Database</a>
        <table class="toc">
          <tr><th>4.1</th><td><a href="#4.1">ODB Query Language</a></td></tr>
          <tr><th>4.2</th><td><a href="#4.2">Parameter Binding</a></td></tr>
          <tr><th>4.3</th><td><a href="#4.3">Executing a Query</a></td></tr>
          <tr><th>4.4</th><td><a href="#4.4">Query Result</a></td></tr>
	  <tr><th>4.5</th><td><a href="#4.5">Prepared Queries</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>5</th><td><a href="#5">Containers</a>
        <table class="toc">
          <tr><th>5.1</th><td><a href="#5.1">Ordered Containers</a></td></tr>
          <tr><th>5.2</th><td><a href="#5.2">Set and Multiset Containers</a></td></tr>
          <tr><th>5.3</th><td><a href="#5.3">Map and Multimap Containers</a></td></tr>
	  <tr>
            <th>5.4</th><td><a href="#5.4">Change-Tracking Containers</a>
              <table class="toc">
                <tr><th>5.4.1</th><td><a href="#5.4.1">Change-Tracking <code>vector</code></a></td></tr>
              </table>
            </td>
          </tr>
          <tr><th>5.5</th><td><a href="#5.5">Using Custom Containers</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>6</th><td><a href="#6">Relationships</a>
        <table class="toc">
          <tr>
            <th>6.1</th><td><a href="#6.1">Unidirectional Relationships</a>
              <table class="toc">
                <tr><th>6.1.1</th><td><a href="#6.1.1">To-One Relationships</a></td></tr>
		<tr><th>6.1.2</th><td><a href="#6.1.2">To-Many Relationships</a></td></tr>
              </table>
            </td>
          </tr>
	  <tr>
            <th>6.2</th><td><a href="#6.2">Bidirectional Relationships</a>
              <table class="toc">
                <tr><th>6.2.1</th><td><a href="#6.2.1">One-to-One Relationships</a></td></tr>
		<tr><th>6.2.2</th><td><a href="#6.2.2">One-to-Many Relationships</a></td></tr>
		<tr><th>6.2.3</th><td><a href="#6.2.3">Many-to-Many Relationships</a></td></tr>
              </table>
            </td>
          </tr>
	  <tr><th>6.3</th><td><a href="#6.3">Circular Relationships</a></td></tr>
          <tr><th>6.4</th><td><a href="#6.4">Lazy Pointers</a></td></tr>
          <tr><th>6.5</th><td><a href="#6.5">Using Custom Smart Pointers</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>7</th><td><a href="#7">Value Types</a>
        <table class="toc">
	  <tr><th>7.1</th><td><a href="#7.1">Simple Value Types</a></td></tr>
	  <tr>
            <th>7.2</th><td><a href="#7.2">Composite Value Types</a>
              <table class="toc">
		<tr><th>7.2.1</th><td><a href="#7.2.1">Composite Object Ids</a></td></tr>
                <tr><th>7.2.2</th><td><a href="#7.2.2">Composite Value Column and Table Names</a></td></tr>
              </table>
            </td>
          </tr>
	  <tr><th>7.3</th><td><a href="#7.3">Pointers and <code>NULL</code> Value Semantics</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>8</th><td><a href="#8">Inheritance</a>
        <table class="toc">
          <tr><th>8.1</th><td><a href="#8.1">Reuse Inheritance</a></td></tr>
	  <tr>
            <th>8.2</th><td><a href="#8.2">Polymorphism Inheritance</a>
              <table class="toc">
		<tr><th>8.2.1</th><td><a href="#8.2.1">Performance and Limitations</a></td></tr>
              </table>
            </td>
          </tr>
	  <tr><th>8.3</th><td><a href="#8.3">Mixed Inheritance</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>9</th><td><a href="#9">Sections</a>
        <table class="toc">
          <tr><th>9.1</th><td><a href="#9.1">Sections and Inheritance</a></td></tr>
	  <tr><th>9.2</th><td><a href="#9.2">Sections and Optimistic Concurrency</a></td></tr>
	  <tr><th>9.3</th><td><a href="#9.3">Sections and Lazy Pointers</a></td></tr>
	  <tr><th>9.4</th><td><a href="#9.4">Sections and Change-Tracking Containers</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>10</th><td><a href="#10">Views</a>
        <table class="toc">
          <tr><th>10.1</th><td><a href="#10.1">Object Views</a></td></tr>
	  <tr><th>10.2</th><td><a href="#10.2">Object Loading Views</a></td></tr>
	  <tr><th>10.3</th><td><a href="#10.3">Table Views</a></td></tr>
	  <tr><th>10.4</th><td><a href="#10.4">Mixed Views</a></td></tr>
	  <tr><th>10.5</th><td><a href="#10.5">View Query Conditions</a></td></tr>
	  <tr><th>10.6</th><td><a href="#10.6">Native Views</a></td></tr>
	  <tr><th>10.7</th><td><a href="#10.7">Other View Features and Limitations</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>11</th><td><a href="#11">Session</a>
        <table class="toc">
          <tr><th>11.1</th><td><a href="#11.1">Object Cache</a></td></tr>
	  <tr><th>11.2</th><td><a href="#11.2">Custom Sessions</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>12</th><td><a href="#12">Optimistic Concurrency</a></td>
    </tr>

    <tr>
      <th>13</th><td><a href="#13">Database Schema Evolution</a>
        <table class="toc">
          <tr><th>13.1</th><td><a href="#13.1">Object Model Version and Changelog</a></td></tr>
	  <tr><th>13.2</th><td><a href="#13.2">Schema Migration</a></td></tr>
	  <tr>
            <th>13.3</th><td><a href="#13.3">Data Migration</a>
              <table class="toc">
		<tr><th>13.3.1</th><td><a href="#13.3.1">Immediate Data Migration</a></td></tr>
		<tr><th>13.3.2</th><td><a href="#13.3.2">Gradual Data Migration</a></td></tr>
              </table>
            </td>
          </tr>
	  <tr>
            <th>13.4</th><td><a href="#13.4">Soft Object Model Changes</a>
              <table class="toc">
		<tr><th>13.4.1</th><td><a href="#13.4.1">Reuse Inheritance Changes</a></td></tr>
		<tr><th>13.4.2</th><td><a href="#13.4.2">Polymorphism Inheritance Changes</a></td></tr>
              </table>
            </td>
          </tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>14</th><td><a href="#14">ODB Pragma Language</a>
        <table class="toc">
          <tr>
            <th>14.1</th><td><a href="#14.1">Object Type Pragmas</a>
              <table class="toc">
                <tr><th>14.1.1</th><td><a href="#14.1.1"><code>table</code></a></td></tr>
		<tr><th>14.1.2</th><td><a href="#14.1.2"><code>pointer</code></a></td></tr>
		<tr><th>14.1.3</th><td><a href="#14.1.3"><code>abstract</code></a></td></tr>
		<tr><th>14.1.4</th><td><a href="#14.1.4"><code>readonly</code></a></td></tr>
		<tr><th>14.1.5</th><td><a href="#14.1.5"><code>optimistic</code></a></td></tr>
		<tr><th>14.1.6</th><td><a href="#14.1.6"><code>no_id</code></a></td></tr>
		<tr><th>14.1.7</th><td><a href="#14.1.7"><code>callback</code></a></td></tr>
		<tr><th>14.1.8</th><td><a href="#14.1.8"><code>schema</code></a></td></tr>
		<tr><th>14.1.9</th><td><a href="#14.1.9"><code>polymorphic</code></a></td></tr>
		<tr><th>14.1.10</th><td><a href="#14.1.10"><code>session</code></a></td></tr>
		<tr><th>14.1.11</th><td><a href="#14.1.11"><code>definition</code></a></td></tr>
		<tr><th>14.1.12</th><td><a href="#14.1.12"><code>transient</code></a></td></tr>
		<tr><th>14.1.13</th><td><a href="#14.1.13"><code>sectionable</code></a></td></tr>
		<tr><th>14.1.14</th><td><a href="#14.1.14"><code>deleted</code></a></td></tr>
		<tr><th>14.1.15</th><td><a href="#14.1.15"><code>bulk</code></a></td></tr>
		<tr><th>14.1.16</th><td><a href="#14.1.16"><code>options</code></a></td></tr>
              </table>
            </td>
          </tr>
          <tr>
            <th>14.2</th><td><a href="#14.2">View Type Pragmas</a>
              <table class="toc">
		<tr><th>14.2.1</th><td><a href="#14.2.1"><code>object</code></a></td></tr>
                <tr><th>14.2.2</th><td><a href="#14.2.2"><code>table</code></a></td></tr>
		<tr><th>14.2.3</th><td><a href="#14.2.3"><code>query</code></a></td></tr>
		<tr><th>14.2.4</th><td><a href="#14.2.4"><code>pointer</code></a></td></tr>
		<tr><th>14.2.5</th><td><a href="#14.2.5"><code>callback</code></a></td></tr>
		<tr><th>14.2.6</th><td><a href="#14.2.6"><code>definition</code></a></td></tr>
		<tr><th>14.2.7</th><td><a href="#14.2.7"><code>transient</code></a></td></tr>
              </table>
            </td>
          </tr>
          <tr>
            <th>14.3</th><td><a href="#14.3">Value Type Pragmas</a>
              <table class="toc">
                <tr><th>14.3.1</th><td><a href="#14.3.1"><code>type</code></a></td></tr>
		<tr><th>14.3.2</th><td><a href="#14.3.2"><code>id_type</code></a></td></tr>
		<tr><th>14.3.3</th><td><a href="#14.3.3"><code>null</code>/<code>not_null</code></a></td></tr>
		<tr><th>14.3.4</th><td><a href="#14.3.4"><code>default</code></a></td></tr>
		<tr><th>14.3.5</th><td><a href="#14.3.5"><code>options</code></a></td></tr>
		<tr><th>14.3.6</th><td><a href="#14.3.6"><code>readonly</code></a></td></tr>
		<tr><th>14.3.7</th><td><a href="#14.3.7"><code>definition</code></a></td></tr>
		<tr><th>14.3.8</th><td><a href="#14.3.8"><code>transient</code></a></td></tr>
		<tr><th>14.3.9</th><td><a href="#14.3.9"><code>unordered</code></a></td></tr>
		<tr><th>14.3.10</th><td><a href="#14.3.10"><code>index_type</code></a></td></tr>
		<tr><th>14.3.11</th><td><a href="#14.3.11"><code>key_type</code></a></td></tr>
		<tr><th>14.3.12</th><td><a href="#14.3.12"><code>value_type</code></a></td></tr>
		<tr><th>14.3.13</th><td><a href="#14.3.13"><code>value_null</code>/<code>value_not_null</code></a></td></tr>
		<tr><th>14.3.14</th><td><a href="#14.3.14"><code>id_options</code></a></td></tr>
		<tr><th>14.3.15</th><td><a href="#14.3.15"><code>index_options</code></a></td></tr>
		<tr><th>14.3.16</th><td><a href="#14.3.16"><code>key_options</code></a></td></tr>
		<tr><th>14.3.17</th><td><a href="#14.3.17"><code>value_options</code></a></td></tr>
		<tr><th>14.3.18</th><td><a href="#14.3.18"><code>id_column</code></a></td></tr>
		<tr><th>14.3.19</th><td><a href="#14.3.19"><code>index_column</code></a></td></tr>
		<tr><th>14.3.20</th><td><a href="#14.3.20"><code>key_column</code></a></td></tr>
		<tr><th>14.3.21</th><td><a href="#14.3.21"><code>value_column</code></a></td></tr>
              </table>
            </td>
          </tr>
          <tr>
            <th>14.4</th><td><a href="#14.4">Data Member Pragmas</a>
              <table class="toc">
                <tr><th>14.4.1</th><td><a href="#14.4.1"><code>id</code></a></td></tr>
                <tr><th>14.4.2</th><td><a href="#14.4.2"><code>auto</code></a></td></tr>
                <tr><th>14.4.3</th><td><a href="#14.4.3"><code>type</code></a></td></tr>
		<tr><th>14.4.4</th><td><a href="#14.4.4"><code>id_type</code></a></td></tr>
		<tr><th>14.4.5</th><td><a href="#14.4.5"><code>get</code>/<code>set</code>/<code>access</code></a></td></tr>
		<tr><th>14.4.6</th><td><a href="#14.4.6"><code>null</code>/<code>not_null</code></a></td></tr>
		<tr><th>14.4.7</th><td><a href="#14.4.7"><code>default</code></a></td></tr>
		<tr><th>14.4.8</th><td><a href="#14.4.8"><code>options</code></a></td></tr>
		<tr><th>14.4.9</th><td><a href="#14.4.9"><code>column</code> (object, composite value)</a></td></tr>
		<tr><th>14.4.10</th><td><a href="#14.4.10"><code>column</code> (view)</a></td></tr>
		<tr><th>14.4.11</th><td><a href="#14.4.11"><code>transient</code></a></td></tr>
		<tr><th>14.4.12</th><td><a href="#14.4.12"><code>readonly</code></a></td></tr>
		<tr><th>14.4.13</th><td><a href="#14.4.13"><code>virtual</code></a></td></tr>
		<tr><th>14.4.14</th><td><a href="#14.4.14"><code>inverse</code></a></td></tr>
		<tr><th>14.4.15</th><td><a href="#14.4.15"><code>on_delete</code></a></td></tr>
		<tr><th>14.4.16</th><td><a href="#14.4.16"><code>version</code></a></td></tr>
		<tr><th>14.4.17</th><td><a href="#14.4.17"><code>index</code></a></td></tr>
		<tr><th>14.4.18</th><td><a href="#14.4.18"><code>unique</code></a></td></tr>
		<tr><th>14.4.19</th><td><a href="#14.4.19"><code>unordered</code></a></td></tr>
		<tr><th>14.4.20</th><td><a href="#14.4.20"><code>table</code></a></td></tr>
		<tr><th>14.4.21</th><td><a href="#14.4.21"><code>load</code>/<code>update</code></a></td></tr>
		<tr><th>14.4.22</th><td><a href="#14.4.22"><code>section</code></a></td></tr>
		<tr><th>14.4.23</th><td><a href="#14.4.23"><code>added</code></a></td></tr>
		<tr><th>14.4.24</th><td><a href="#14.4.24"><code>deleted</code></a></td></tr>
		<tr><th>14.4.25</th><td><a href="#14.4.25"><code>index_type</code></a></td></tr>
		<tr><th>14.4.26</th><td><a href="#14.4.26"><code>key_type</code></a></td></tr>
		<tr><th>14.4.27</th><td><a href="#14.4.27"><code>value_type</code></a></td></tr>
		<tr><th>14.4.28</th><td><a href="#14.4.28"><code>value_null</code>/<code>value_not_null</code></a></td></tr>
		<tr><th>14.4.29</th><td><a href="#14.4.29"><code>id_options</code></a></td></tr>
		<tr><th>14.4.30</th><td><a href="#14.4.30"><code>index_options</code></a></td></tr>
		<tr><th>14.4.31</th><td><a href="#14.4.31"><code>key_options</code></a></td></tr>
		<tr><th>14.4.32</th><td><a href="#14.4.32"><code>value_options</code></a></td></tr>
		<tr><th>14.4.33</th><td><a href="#14.4.33"><code>id_column</code></a></td></tr>
		<tr><th>14.4.34</th><td><a href="#14.4.34"><code>index_column</code></a></td></tr>
		<tr><th>14.4.35</th><td><a href="#14.4.35"><code>key_column</code></a></td></tr>
		<tr><th>14.4.36</th><td><a href="#14.4.36"><code>value_column</code></a></td></tr>
		<tr><th>14.4.36</th><td><a href="#14.4.37"><code>points_to</code></a></td></tr>
              </table>
            </td>
          </tr>
	  <tr>
            <th>14.5</th><td><a href="#14.5">Namespace Pragmas</a>
              <table class="toc">
		<tr><th>14.5.1</th><td><a href="#14.5.1"><code>pointer</code></a></td></tr>
		<tr><th>14.5.2</th><td><a href="#14.5.2"><code>table</code></a></td></tr>
                <tr><th>14.5.3</th><td><a href="#14.5.3"><code>schema</code></a></td></tr>
		<tr><th>14.5.4</th><td><a href="#14.5.4"><code>session</code></a></td></tr>
              </table>
            </td>
          </tr>
	  <tr>
            <th>14.6</th><td><a href="#14.6">Object Model Pragmas</a>
              <table class="toc">
		<tr><th>14.6.1</th><td><a href="#14.6.1"><code>version</code></a></td></tr>
              </table>
            </td>
          </tr>
          <tr>
            <th>14.7</th><td><a href="#14.7">Index Definition Pragmas</a></td>
          </tr>
          <tr>
            <th>14.8</th><td><a href="#14.8">Type Mapping Pragmas</a>
              <table class="toc">
		<tr><th>14.8.1</th><td><a href="#14.8.1">C++ Type Mapping Pragmas</a></td></tr>
		<tr><th>14.8.2</th><td><a href="#14.8.2">Database Type Mapping Pragmas</a></td></tr>
              </table>
	    </td>
          </tr>
          <tr>
            <th>14.9</th><td><a href="#14.9">C++ Compiler Warnings</a>
              <table class="toc">
                <tr><th>14.9.1</th><td><a href="#14.9.1">GNU C++</a></td></tr>
                <tr><th>14.9.2</th><td><a href="#14.9.2">Visual C++</a></td></tr>
                <tr><th>14.9.3</th><td><a href="#14.9.3">Sun C++</a></td></tr>
		<tr><th>14.9.4</th><td><a href="#14.9.4">IBM XL C++</a></td></tr>
		<tr><th>14.9.5</th><td><a href="#14.9.5">HP aC++</a></td></tr>
		<tr><th>14.9.6</th><td><a href="#14.9.6">Clang</a></td></tr>
              </table>
            </td>
          </tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>15</th><td><a href="#15">Advanced Techniques and Mechanisms</a>
        <table class="toc">
          <tr><th>15.1</th><td><a href="#15.1">Transaction Callbacks</a></td></tr>
	  <tr><th>15.2</th><td><a href="#15.2">Persistent Class Template Instantiations</a></td></tr>
	  <tr><th>15.3</th><td><a href="#15.3">Bulk Database Operations</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th colspan="2"><a href="#II">PART II DATABASE SYSTEMS</a></th>
    </tr>

    <tr>
      <th>16</th><td><a href="#16">Multi-Database Support</a>
        <table class="toc">
          <tr><th>16.1</th><td><a href="#16.1">Static Multi-Database Support</a></td></tr>
          <tr>
            <th>16.2</th><td><a href="#16.2">Dynamic Multi-Database Support</a>
              <table class="toc">
                <tr><th>16.2.2</th><td><a href="#16.2.2">16.2.2 Dynamic Loading of Database Support Code</a></td></tr>
              </table>
            </td>
          </tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>17</th><td><a href="#17">MySQL Database</a>
        <table class="toc">
          <tr>
            <th>17.1</th><td><a href="#17.1">MySQL Type Mapping</a>
              <table class="toc">
                <tr><th>17.1.1</th><td><a href="#17.1.1">String Type Mapping</a></td></tr>
                <tr><th>17.1.2</th><td><a href="#17.1.2">Binary Type Mapping</a></td></tr>
                <tr><th>17.1.3</th><td><a href="#17.1.3">Mixed Automatic/0 Object Id Assignment</a></td></tr>
              </table>
            </td>
          </tr>
          <tr><th>17.2</th><td><a href="#17.2">MySQL Database Class</a></td></tr>
          <tr><th>17.3</th><td><a href="#17.3">MySQL Connection and Connection Factory</a></td></tr>
	  <tr><th>17.4</th><td><a href="#17.4">MySQL Exceptions</a></td></tr>
	  <tr>
            <th>17.5</th><td><a href="#17.5">MySQL Limitations</a>
              <table class="toc">
                <tr><th>17.5.1</th><td><a href="#17.5.1">Foreign Key Constraints</a></td></tr>
              </table>
            </td>
          </tr>
          <tr><th>17.6</th><td><a href="#17.6">MySQL Index Definition</a></td></tr>
	  <tr><th>17.7</th><td><a href="#17.7">MySQL Stored Procedures</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>18</th><td><a href="#18">SQLite Database</a>
        <table class="toc">
          <tr>
            <th>18.1</th><td><a href="#18.1">SQLite Type Mapping</a>
              <table class="toc">
                <tr><th>18.1.1</th><td><a href="#18.1.1">String Type Mapping</a></td></tr>
                <tr><th>18.1.2</th><td><a href="#18.1.2">Binary Type Mapping</a></td></tr>
		<tr><th>18.1.3</th><td><a href="#18.1.3">Incremental <code>BLOB</code>/<code>TEXT</code> I/O</a></td></tr>
		<tr><th>18.1.4</th><td><a href="#18.1.4">Mixed Automatic/Manual Object Id Assignment</a></td></tr>
              </table>
            </td>
          </tr>
          <tr><th>18.2</th><td><a href="#18.2">SQLite Database Class</a></td></tr>
          <tr><th>18.3</th><td><a href="#18.3">SQLite Connection and Connection Factory</a></td></tr>
	  <tr><th>18.4</th><td><a href="#18.4">Attached SQLite Databases</a></td></tr>
	  <tr><th>18.5</th><td><a href="#18.5">SQLite Exceptions</a></td></tr>
          <tr>
            <th>18.6</th><td><a href="#18.6">SQLite Limitations</a>
              <table class="toc">
                <tr><th>18.6.1</th><td><a href="#18.6.1">Query Result Caching</a></td></tr>
		<tr><th>18.6.2</th><td><a href="#18.6.2">Automatic Assignment of Object Ids</a></td></tr>
		<tr><th>18.6.3</th><td><a href="#18.6.3">Foreign Key Constraints</a></td></tr>
		<tr><th>18.6.4</th><td><a href="#18.6.4">Constraint Violations</a></td></tr>
		<tr><th>18.6.5</th><td><a href="#18.6.5">Sharing of Queries</a></td></tr>
		<tr><th>18.6.6</th><td><a href="#18.6.6">Forced Rollback</a></td></tr>
		<tr><th>18.6.7</th><td><a href="#18.6.7">Database Schema Evolution</a></td></tr>
              </table>
            </td>
          </tr>
          <tr><th>18.7</th><td><a href="#18.7">SQLite Index Definition</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>19</th><td><a href="#19">PostgreSQL Database</a>
        <table class="toc">
          <tr>
            <th>19.1</th><td><a href="#19.1">PostgreSQL Type Mapping</a>
              <table class="toc">
                <tr><th>19.1.1</th><td><a href="#19.1.1">String Type Mapping</a></td></tr>
                <tr><th>19.1.2</th><td><a href="#19.1.2">Binary Type and <code>UUID</code> Mapping</a></td></tr>
              </table>
            </td>
          </tr>
          <tr><th>19.2</th><td><a href="#19.2">PostgreSQL Database Class</a></td></tr>
          <tr><th>19.3</th><td><a href="#19.3">PostgreSQL Connection and Connection Factory</a></td></tr>
	  <tr><th>19.4</th><td><a href="#19.4">PostgreSQL Exceptions</a></td></tr>
          <tr>
            <th>19.5</th><td><a href="#19.5">PostgreSQL Limitations</a>
              <table class="toc">
                <tr><th>19.5.1</th><td><a href="#19.5.1">Query Result Caching</a></td></tr>
                <tr><th>19.5.2</th><td><a href="#19.5.2">Foreign Key Constraints</a></td></tr>
		<tr><th>19.5.3</th><td><a href="#19.5.3">Unique Constraint Violations</a></td></tr>
		<tr><th>19.5.4</th><td><a href="#19.5.4">Date-Time Format</a></td></tr>
		<tr><th>19.5.5</th><td><a href="#19.5.5">Timezones</a></td></tr>
		<tr><th>19.5.6</th><td><a href="#19.5.6"><code>NUMERIC</code> Type Support</a></td></tr>
		<tr><th>19.5.7</th><td><a href="#19.5.7">Bulk Operations Support</a></td></tr>
              </table>
            </td>
          </tr>
	  <tr><th>19.6</th><td><a href="#19.6">PostgreSQL Index Definition</a></td></tr>
	  <tr><th>19.7</th><td><a href="#19.7">PostgreSQL Stored Procedures and Functions</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>20</th><td><a href="#20">Oracle Database</a>
        <table class="toc">
          <tr>
            <th>20.1</th><td><a href="#20.1">Oracle Type Mapping</a>
              <table class="toc">
                <tr><th>20.1.1</th><td><a href="#20.1.1">String Type Mapping</a></td></tr>
                <tr><th>20.1.2</th><td><a href="#20.1.2">Binary Type Mapping</a></td></tr>
              </table>
            </td>
          </tr>
          <tr><th>20.2</th><td><a href="#20.2">Oracle Database Class</a></td></tr>
          <tr><th>20.3</th><td><a href="#20.3">Oracle Connection and Connection Factory</a></td></tr>
	  <tr><th>20.4</th><td><a href="#20.4">Oracle Exceptions</a></td></tr>
          <tr>
            <th>20.5</th><td><a href="#20.5">Oracle Limitations</a>
              <table class="toc">
                <tr><th>20.5.1</th><td><a href="#20.5.1">Identifier Truncation</a></td></tr>
		<tr><th>20.5.2</th><td><a href="#20.5.2">Query Result Caching</a></td></tr>
		<tr><th>20.5.3</th><td><a href="#20.5.3">Foreign Key Constraints</a></td></tr>
		<tr><th>20.5.4</th><td><a href="#20.5.4">Unique Constraint Violations</a></td></tr>
		<tr><th>20.5.5</th><td><a href="#20.5.5">Large <code>FLOAT</code> and <code>NUMBER</code> Types</a></td></tr>
		<tr><th>20.5.6</th><td><a href="#20.5.6">Timezones</a></td></tr>
		<tr><th>20.5.7</th><td><a href="#20.5.7"><code>LONG</code> Types</a></td></tr>
		<tr><th>20.5.8</th><td><a href="#20.5.8">LOB Types and By-Value Accessors/Modifiers</a></td></tr>
		<tr><th>20.5.9</th><td><a href="#20.5.9">Database Schema Evolution</a></td></tr>
              </table>
            </td>
          </tr>
	  <tr><th>20.6</th><td><a href="#20.6">Oracle Index Definition</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>21</th><td><a href="#21">Microsoft SQL Server Database</a>
        <table class="toc">
          <tr>
            <th>21.1</th><td><a href="#21.1">SQL Server Type Mapping</a>
              <table class="toc">
                <tr><th>21.1.1</th><td><a href="#21.1.1">String Type Mapping</a></td></tr>
                <tr><th>21.1.2</th><td><a href="#21.1.2">Binary Type and <code>UNIQUEIDENTIFIER</code> Mapping</a></td></tr>
		<tr><th>21.1.3</th><td><a href="#21.1.3"><code>ROWVERSION</code> Mapping</a></td></tr>
		<tr><th>21.1.4</th><td><a href="#21.1.4">Long String and Binary Types</a></td></tr>
              </table>
            </td>
          </tr>
          <tr><th>21.2</th><td><a href="#21.2">SQL Server Database Class</a></td></tr>
          <tr><th>21.3</th><td><a href="#21.3">SQL Server Connection and Connection Factory</a></td></tr>
	  <tr><th>21.4</th><td><a href="#21.4">SQL Server Exceptions</a></td></tr>
          <tr>
            <th>21.5</th><td><a href="#21.5">SQL Server Limitations</a>
              <table class="toc">
                <tr><th>21.5.1</th><td><a href="#21.5.1">Query Result Caching</a></td></tr>
		<tr><th>21.5.2</th><td><a href="#21.5.2">Foreign Key Constraints</a></td></tr>
		<tr><th>21.5.3</th><td><a href="#21.5.3">Unique Constraint Violations</a></td></tr>
		<tr><th>21.5.4</th><td><a href="#21.5.4">Multi-threaded Windows Applications</a></td></tr>
		<tr><th>21.5.5</th><td><a href="#21.5.5">Affected Row Count and DDL Statements</a></td></tr>
		<tr><th>21.5.6</th><td><a href="#21.5.6">Long Data and Auto Object Ids, <code>ROWVERSION</code></a></td></tr>
		<tr><th>21.5.7</th><td><a href="#21.5.7">Long Data and By-Value Accessors/Modifiers</a></td></tr>
		<tr><th>21.5.8</th><td><a href="#21.5.8">Bulk Update and <code>ROWVERSION</code></a></td></tr>
              </table>
            </td>
          </tr>
          <tr><th>21.6</th><td><a href="#21.6">SQL Server Index Definition</a></td></tr>
	  <tr><th>21.7</th><td><a href="#21.7">SQL Server Stored Procedures</a></td></tr>
        </table>
      </td>
    </tr>

    <tr>
      <th colspan="2"><a href="#III">PART III PROFILES</a></th>
    </tr>

    <tr>
      <th>22</th><td><a href="#22">Profiles Introduction</a></td>
    </tr>

    <tr>
      <th>23</th><td><a href="#23">Boost Profile</a>
        <table class="toc">
          <tr><th>23.1</th><td><a href="#23.1">Smart Pointers Library</a></td></tr>
          <tr><th>23.2</th><td><a href="#23.2">Unordered Containers Library</a></td></tr>
	  <tr><th>23.3</th><td><a href="#23.3">Multi-Index Container Library</a></td></tr>
	  <tr><th>23.4</th><td><a href="#23.4">Optional Library</a></td></tr>
          <tr>
	    <th>23.5</th><td><a href="#23.5">Date Time Library</a>
	      <table class="toc">
	        <tr><th>23.5.1</th><td><a href="#23.5.1">MySQL Database Type Mapping</a></td></tr>
		<tr><th>23.5.2</th><td><a href="#23.5.2">SQLite Database Type Mapping</a></td></tr>
		<tr><th>23.5.3</th><td><a href="#23.5.3">PostgreSQL Database Type Mapping</a></td></tr>
		<tr><th>23.5.4</th><td><a href="#23.5.4">Oracle Database Type Mapping</a></td></tr>
		<tr><th>23.5.5</th><td><a href="#23.5.5">SQL Server Database Type Mapping</a></td></tr>
	      </table>
	    </td>
	  </tr>
          <tr>
	    <th>23.6</th><td><a href="#23.6">Uuid Library</a>
	      <table class="toc">
	        <tr><th>23.6.1</th><td><a href="#23.6.1">MySQL Database Type Mapping</a></td></tr>
		<tr><th>23.6.2</th><td><a href="#23.6.2">SQLite Database Type Mapping</a></td></tr>
		<tr><th>23.6.3</th><td><a href="#23.6.3">PostgreSQL Database Type Mapping</a></td></tr>
		<tr><th>23.6.4</th><td><a href="#23.6.4">Oracle Database Type Mapping</a></td></tr>
		<tr><th>23.6.5</th><td><a href="#23.6.5">SQL Server Database Type Mapping</a></td></tr>
	      </table>
	    </td>
	  </tr>
        </table>
      </td>
    </tr>

    <tr>
      <th>24</th><td><a href="#24">Qt Profile</a>
        <table class="toc">
          <tr>
	    <th>24.1</th><td><a href="#24.1">Basic Types Library</a>
	      <table class="toc">
	        <tr><th>24.1.1</th><td><a href="#24.1.1">MySQL Database Type Mapping</a></td></tr>
		<tr><th>24.1.2</th><td><a href="#24.1.2">SQLite Database Type Mapping</a></td></tr>
		<tr><th>24.1.3</th><td><a href="#24.1.3">PostgreSQL Database Type Mapping</a></td></tr>
		<tr><th>24.1.4</th><td><a href="#24.1.4">Oracle Database Type Mapping</a></td></tr>
		<tr><th>24.1.5</th><td><a href="#24.1.5">SQL Server Database Type Mapping</a></td></tr>
              </table>
	    </td>
	  </tr>
          <tr><th>24.2</th><td><a href="#24.2">Smart Pointers Library</a></td></tr>
          <tr>
	    <th>24.3</th><td><a href="#24.3">Containers Library</a>
	      <table class="toc">
	        <tr><th>24.3.1</th><td><a href="#24.3.1">Change-Tracking <code>QList</code></a></td></tr>
              </table>
	    </td>
	  </tr>
          <tr>
	    <th>24.4</th><td><a href="#24.4">Date Time Library</a>
	      <table class="toc">
	        <tr><th>24.4.1</th><td><a href="#24.4.1">MySQL Database Type Mapping</a></td></tr>
		<tr><th>24.4.2</th><td><a href="#24.4.2">SQLite Database Type Mapping</a></td></tr>
		<tr><th>24.4.3</th><td><a href="#24.4.3">PostgreSQL Database Type Mapping</a></td></tr>
		<tr><th>24.4.4</th><td><a href="#24.4.4">Oracle Database Type Mapping</a></td></tr>
		<tr><th>24.4.5</th><td><a href="#24.4.5">SQL Server Database Type Mapping</a></td></tr>
	      </table>
	    </td>
	  </tr>
        </table>
      </td>
    </tr>

  </table>
  </div>

  <hr class="page-break"/>
  <h1><a name="0">Preface</a></h1>

  <p>As more critical aspects of our lives become dependant on software
     systems, more and more applications are required to save the data
     they work on in persistent and reliable storage. Database management
     systems and, in particular, relational database management systems
     (RDBMS) are commonly used for such storage. However, while the
     application development techniques and programming languages have
     evolved significantly over the past decades, the relational database
     technology in this area stayed relatively unchanged. In particular,
     this led to the now infamous mismatch between the object-oriented
     model used by many modern applications and the relational model still
     used by RDBMS.</p>

  <p>While relational databases may be inconvenient to use from modern
     programming languages, they are still the main choice for many
     applications due to their maturity, reliability, as well as the
     availability of tools and alternative implementations.</p>

  <p>To allow application developers to utilize relational databases
     from their object-oriented applications, a technique called
     object-relational mapping (ORM) is often used. It involves a
     conversion layer that maps between objects in the application's
     memory and their relational representation in the database. While
     the object-relational mapping code can be written manually,
     automated ORM systems are available for most object-oriented
     programming languages in use today.</p>

  <p>ODB is an ORM system for the C++ programming language. It was
     designed and implemented with the following main goals:</p>

  <ul class="list">
    <li>Provide a fully-automatic ORM system. In particular, the
        application developer should not have to manually write any
        mapping code, neither for persistent classes nor for their
        data member. </li>

    <li>Provide clean and easy to use object-oriented persistence
        model and database APIs that support the development of realistic
        applications for a wide variety of domains.</li>

    <li>Provide a portable and thread-safe implementation. ODB should be
        written in standard C++ and capable of persisting any standard
        C++ classes.</li>

    <li>Provide profiles that integrate ODB with type systems of
        widely-used frameworks and libraries such as Qt and Boost.</li>

    <li>Provide a high-performance and low overhead implementation. ODB
        should make efficient use of database and application resources.</li>

  </ul>


  <h2><a name="0.1">About This Document</a></h2>

  <p>The goal of this manual is to provide you with an understanding
     of the object persistence model and APIs which are implemented by ODB.
     As such, this document is intended for C++ application developers and
     software architects who are looking for a C++ object persistence
     solution. Prior experience with C++ is required to understand
     this document. A basic understanding of relational database systems
     is advantageous but not expected or required.</p>


  <h2><a name="0.2">More Information</a></h2>

  <p>Beyond this manual, you may also find the following sources of
     information useful:</p>

  <ul class="list">
    <li><a href="http://www.codesynthesis.com/products/odb/doc/odb.xhtml">ODB
        Compiler Command Line Manual.</a></li>

    <li>The <code>INSTALL</code> files in the ODB source packages provide
        build instructions for various platforms.</li>

    <li>The <code>odb-examples</code> package contains a collection of
        examples and a README file with an overview of each example.</li>

    <li>The <a href="http://www.codesynthesis.com/mailman/listinfo/odb-users">odb-users</a>
        mailing list is the place to ask technical questions about ODB.
        Furthermore, the searchable
        <a href="http://www.codesynthesis.com/pipermail/odb-users/">archives</a>
        may already have answers to some of your questions.</li>

  </ul>


  <!-- PART -->


  <hr class="page-break"/>
  <h1><a name="I">PART I&nbsp;&nbsp;
      <span style="font-weight: normal;">OBJECT-RELATIONAL MAPPING</span></a></h1>

  <p>Part I describes the essential database concepts, APIs, and tools that
     together comprise the object-relational mapping for C++ as implemented
     by ODB. It consists of the following chapters.</p>

  <table class="toc">
    <tr><th>1</th><td><a href="#1">Introduction</a></td></tr>
    <tr><th>2</th><td><a href="#2">Hello World Example</a></td></tr>
    <tr><th>3</th><td><a href="#3">Working with Persistent Objects</a></td></tr>
    <tr><th>4</th><td><a href="#4">Querying the Database</a></td></tr>
    <tr><th>5</th><td><a href="#5">Containers</a></td></tr>
    <tr><th>6</th><td><a href="#6">Relationships</a></td></tr>
    <tr><th>7</th><td><a href="#7">Value Types</a></td></tr>
    <tr><th>8</th><td><a href="#8">Inheritance</a></td></tr>
    <tr><th>10</th><td><a href="#10">Views</a></td></tr>
    <tr><th>11</th><td><a href="#11">Session</a></td></tr>
    <tr><th>12</th><td><a href="#12">Optimistic Concurrency</a></td></tr>
    <tr><th>13</th><td><a href="#13">Database Schema Evolution</a></td></tr>
    <tr><th>14</th><td><a href="#14">ODB Pragma Language</a></td></tr>
  </table>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="1">1 Introduction</a></h1>

  <p>ODB is an object-relational mapping (ORM) system for C++. It provides
     tools, APIs, and library support that allow you to persist C++ objects
     to a relational database (RDBMS) without having to deal with tables,
     columns, or SQL and without manually writing any of the mapping code.</p>

  <p>ODB is highly flexible and customizable. It can either completely
     hide the relational nature of the underlying database or expose
     some of the details as required. For example, you can automatically
     map basic C++ types to suitable SQL types, generate the relational
     database schema for your persistent classes, and use simple, safe,
     and yet powerful object query language instead of SQL. Or you can
     assign SQL types to individual data members, use the existing
     database schema, run native SQL <code>SELECT</code> queries, and
     call stored procedures. In fact, at an extreme, ODB can be used
     as <em>just</em> a convenient way to handle results of native SQL
     queries.</p>

  <p>ODB is not a framework. It does not dictate how you should write
     your application. Rather, it is designed to fit into your
     style and architecture by only handling object persistence
     and not interfering with any other functionality. There is
     no common base type that all persistent classes should derive
     from nor are there any restrictions on the data member types
     in persistent classes. Existing classes can be made persistent
     with a few or no modifications.</p>

  <p>ODB has been designed for high performance and low memory
     overhead. Prepared statements are used to send and receive
     object state in binary format instead of text which reduces
     the load on the application and the database server. Extensive
     caching of connections, prepared statements, and buffers saves
     time and resources on connection establishment, statement parsing,
     and memory allocations. For each supported database system the
     native C API is used instead of ODBC or higher-level wrapper
     APIs to reduce overhead and provide the most efficient implementation
     for each database operation. Finally, persistent classes have
     zero memory overhead. There are no hidden "database" members
     that each class must have nor are there per-object data structures
     allocated by ODB.</p>

  <p>In this chapter we present a high-level overview of ODB.
     We will start with the ODB architecture and then outline the
     workflow of building an application that uses ODB. We will
     then continue by contrasting the drawbacks of the traditional
     way of saving C++ objects to relational databases with the
     benefits of using ODB for object persistence. We conclude the
     chapter by discussing the C++ standards supported by ODB. The
     next chapter takes a more hands-on approach and shows the
     concrete steps necessary to implement object persistence in
     a simple "Hello World" application.</p>

  <h2><a name="1.1">1.1 Architecture and Workflow</a></h2>

  <p>From the application developer's perspective, ODB
     consists of three main components: the ODB compiler, the common
     runtime library, called <code>libodb</code>, and the
     database-specific runtime libraries, called
     <code>libodb-&lt;database></code>, where &lt;database> is
     the name of the database system  this runtime
     is for, for example, <code>libodb-mysql</code>. For instance,
     if the application is going to use the MySQL database for
     object persistence, then the three ODB components that this
     application will use are the ODB compiler, <code>libodb</code>
     and <code>libodb-mysql</code>.</p>

  <p>The ODB compiler generates the database support code for
     persistent classes in your application. The input to the ODB
     compiler is one or more C++ header files defining C++ classes
     that you want to make persistent. For each input header file
     the ODB compiler generates a set of C++ source files implementing
     conversion between persistent C++ classes defined in this
     header and their database representation. The ODB compiler
     can also generate a database schema file that creates tables
     necessary to store the persistent classes.</p>

  <p>The ODB compiler is a real C++ compiler except that it produces
     C++ instead of assembly or machine code. In particular, it is not
     an ad-hoc header pre-processor that is only capable of recognizing
     a subset of C++. ODB is capable of parsing any standard C++ code.</p>

  <p>The common runtime library defines database system-independent
     interfaces that your application can use to manipulate persistent
     objects. The database-specific runtime library provides implementations
     of these interfaces for a concrete database as well as other
     database-specific utilities that are used by the generated code.
     Normally, the application does not use the database-specific
     runtime library directly but rather works with it via the common
     interfaces from <code>libodb</code>. The following diagram shows
     the object persistence architecture of an application that uses
     MySQL as the underlying database system:</p>

  <!-- align=center is needed for html2ps -->
  <div class="img" align="center"><img src="odb-arch.png"/></div>

  <p>The ODB system also defines two special-purpose languages:
     the ODB Pragma Language and ODB Query Language. The ODB Pragma
     Language is used to communicate various properties of persistent
     classes to the ODB compiler by means of special <code>#pragma</code>
     directives embedded in the C++ header files. It controls aspects
     of the object-relational mapping such as names of tables and columns
     that are used for persistent classes and their members or mapping between
     C++ types and database types.</p>

  <p>The ODB Query Language is an object-oriented database query
     language that can be used to search for objects matching
     certain criteria. It is modeled after and is integrated into
     C++ allowing you to write expressive and safe queries that look
     and feel like ordinary C++.</p>

  <p>The use of the ODB compiler to generate database support code
     adds an additional step to your application build sequence. The
     following diagram outlines the typical build workflow of an
     application that uses ODB:</p>

  <!-- align=center is needed for html2ps -->
  <div class="img" align="center"><img src="odb-flow.png"/></div>

  <h2><a name="1.2">1.2 Benefits</a></h2>

  <p>The traditional way of saving C++ objects to relational databases
     requires that you manually write code which converts between the database
     and C++ representations of each persistent class. The actions that
     such code usually performs include conversion between C++ values and
     strings or database types, preparation and execution of SQL queries,
     as well as handling the result sets. Writing this code manually has
     the following drawbacks:</p>

  <ul class="list">
    <li><b>Difficult and time consuming.</b> Writing database conversion
        code for any non-trivial application requires extensive
        knowledge of the specific database system and its APIs.
        It can also take a considerable amount of time to write
        and maintain. Supporting multi-threaded applications can
        complicate this task even further.</li>

    <li><b>Suboptimal performance.</b> Optimal conversion often
        requires writing large amounts of extra code, such as
        parameter binding for prepared statements and caching
        of connections, statements, and buffers. Writing code
        like this in an ad-hoc manner is often too difficult
        and time consuming.</li>

    <li><b>Database vendor lock-in.</b> The conversion code is written for
        a specific database which makes it hard to switch to another
        database vendor.</li>

    <li><b>Lack of type safety.</b> It is easy to misspell column names or
        pass incompatible values in SQL queries. Such errors will
        only be detected at runtime.</li>

    <li><b>Complicates the application.</b> The database conversion code
        often ends up interspersed throughout the application making it
        hard to debug, change, and maintain.</li>
  </ul>

  <p>In contrast, using ODB for C++ object persistence has the
     following benefits:</p>

  <ul class="list">
    <li><b>Ease of use.</b> ODB automatically generates database conversion
        code from your C++ class declarations and allows you to manipulate
        persistent objects using simple and thread-safe object-oriented
        database APIs.</li>

    <li><b>Concise code.</b> With ODB hiding the details of the underlying
        database, the application logic is written using the natural object
        vocabulary instead of tables, columns and SQL. The resulting code
        is simpler and thus easier to read and understand.</li>

    <li><b>Optimal performance.</b> ODB has been designed for high performance
        and low memory overhead. All the available optimization techniques,
        such as prepared statements and extensive connection, statement,
        and buffer caching, are used to provide the most efficient
        implementation for each database operation.</li>

    <li><b>Database portability.</b> Because the database conversion code
        is automatically generated, it is easy to switch from one database
        vendor to another. In fact, it is possible to test your application
        on several database systems before making a choice.</li>

    <li><b>Safety.</b> The ODB object persistence and query APIs are
        statically typed. You use C++ identifiers instead of strings
        to refer to object members and the generated code makes sure
        database and C++ types are compatible. All this helps catch
        programming errors at compile-time rather than at runtime.</li>

    <li><b>Maintainability.</b> Automatic code generation minimizes the
        effort needed to adapt the application to changes in persistent
        classes. The database support code is kept separately from the
        class declarations and application logic. This makes the
        application easier to debug and maintain.</li>
  </ul>

  <p>Overall, ODB provides an easy to use yet flexible and powerful
     object-relational mapping (ORM) system for C++. Unlike other
     ORM implementations for C++ that still require you to write
     database conversion or member registration code for each
     persistent class, ODB keeps persistent classes purely
     declarative. The functional part, the database conversion
     code, is automatically generated by the ODB compiler from
     these declarations.</p>

  <h2><a name="1.3">1.3 Supported C++ Standards</a></h2>

  <p>ODB provides support for ISO/IEC C++ 1998/2003 (C++98/03), ISO/IEC C++
     2011 (C++11), as well later standards with the majority of the examples
     in this manual using C++11. The <code>c++11</code> example in the
     <code>odb-examples</code> package shows ODB support for various
     features new in C++11.</p>

  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="2">2 Hello World Example</a></h1>

  <p>In this chapter we will show how to create a simple C++
     application that relies on ODB for object persistence using
     the traditional "Hello World" example. In particular, we will
     discuss how to declare persistent classes, generate database
     support code, as well as compile and run our application. We
     will also learn how to make objects persistent, load, update
     and delete persistent objects, as well as query the database
     for persistent objects that match certain criteria. The example
     also shows how to define and use views, a mechanism that allows
     us to create projections of persistent objects, database tables,
     or to handle results of native SQL queries or stored procedure
     calls.</p>

  <p>The code presented in this chapter is based on the
     <code>hello</code> example which can be found in the
     <code>odb-examples</code> package of the ODB distribution.</p>

  <h2><a name="2.1">2.1 Declaring Persistent Classes</a></h2>

  <p>In our "Hello World" example we will depart slightly from
     the norm and say hello to people instead of the world. People
     in our application will be represented as objects of C++ class
     <code>person</code> which is saved in <code>person.hxx</code>:</p>

  <pre class="cxx">
// person.hxx
//

#include &lt;string>

class person
{
public:
  person (const std::string&amp; first,
          const std::string&amp; last,
          unsigned short age);

  const std::string&amp; first () const;
  const std::string&amp; last () const;

  unsigned short age () const;
  void age (unsigned short);

private:
  std::string first_;
  std::string last_;
  unsigned short age_;
};
  </pre>

  <p>In order not to miss anyone whom we need to greet, we would like
  to save the <code>person</code> objects in a database. To achieve this
  we declare the <code>person</code> class as persistent:</p>

  <pre class="cxx">
// person.hxx
//

#include &lt;string>

#include &lt;odb/core.hxx>     // (1)

#pragma db object           // (2)
class person
{
  ...

private:
  person () {}              // (3)

  friend class odb::access; // (4)

  #pragma db id auto        // (5)
  unsigned long long id_;   // (5)

  std::string first_;
  std::string last_;
  unsigned short age_;
};
  </pre>

  <p>To be able to save the <code>person</code> objects in the database
     we had to make five changes, marked with (1) to (5), to the original
     class definition. The first change is the inclusion of the ODB
     header <code>&lt;odb/core.hxx></code>. This header provides a number
     of core ODB declarations, such as <code>odb::access</code>, that
     are used to define persistent classes.</p>

  <p>The second change is the addition of <code>db&nbsp;object</code>
     pragma just before the class definition. This pragma tells the
     ODB compiler that the class that follows is persistent. Note
     that making a class persistent does not mean that all objects
     of this class will automatically be stored in the database.
     You would still create ordinary or <em>transient</em> instances
     of this class just as you would before. The difference is that
     now you can make such transient instances persistent, as we will
     see shortly.</p>

  <p>The third change is the addition of the default constructor.
     The ODB-generated database support code will use this constructor
     when instantiating an object from the persistent state. Just as we have
     done for the <code>person</code> class, you can make the default
     constructor private or protected if you don't want to make it
     available to the users of your class. Note also that with some
     limitations it is possible to have a persistent class without
     the default constructor.</p>

  <p>With the fourth change we make the <code>odb::access</code> class a
     friend of our <code>person</code> class. This is necessary to make
     the default constructor and the data members accessible to the
     database support code. If your class has a public default constructor and
     either public data members or public accessors and modifiers for the
     data members, then the <code>friend</code> declaration is unnecessary.</p>

  <p>The final change adds a data member called <code>id_</code> which
     is preceded by another pragma. In ODB every persistent object normally
     has a unique, within its class, identifier. Or, in other words, no two
     persistent instances of the same type have equal identifiers. While it
     is possible to define a persistent class without an object id, the number
     of database operations that can be performed on such a class is limited.
     For our class we use an integer id. The <code>db&nbsp;id auto</code>
     pragma that precedes the <code>id_</code> member tells the ODB compiler
     that the following member is the object's identifier. The
     <code>auto</code> specifier indicates that it is a database-assigned
     id. A unique id will be automatically generated by the database and
     assigned to the object when it is made persistent.</p>

  <p>In this example we chose to add an identifier because none of
     the existing members could serve the same purpose. However, if
     a class already has a member with suitable properties, then it
     is natural to use that member as an identifier. For example,
     if our <code>person</code> class contained some form of personal
     identification (SSN in the United States or ID/passport number
     in other countries), then we could use that as an id. Or, if
     we stored an email associated with each person, then we could
     have used that if each person is presumed to have a unique
     email address.</p>

  <p>As another example, consider the following alternative version
     of the <code>person</code> class. Here we use one of
     the existing data members as id. Also the data members are kept
     private and are instead accessed via public accessor and modifier
     functions. Finally, the ODB pragmas are grouped together and are
     placed after the class definition. They could have also been moved
     into a separate header leaving the original class completely
     unchanged (for more information on such a non-intrusive conversion
     refer to <a href="#14">Chapter 14, "ODB Pragma Language"</a>).</p>

  <pre class="cxx">
class person
{
public:
  person ();

  const std::string&amp; email () const;
  void email (const std::string&amp;);

  const std::string&amp; get_name () const;
  std::string&amp; set_name ();

  unsigned short getAge () const;
  void setAge (unsigned short);

private:
  std::string email_;
  std::string name_;
  unsigned short age_;
};

#pragma db object(person)
#pragma db member(person::email_) id
  </pre>

  <p>Now that we have the header file with the persistent class, let's
     see how we can generate that database support code.</p>

  <h2><a name="2.2">2.2 Generating Database Support Code</a></h2>

  <p>The persistent class definition that we created in the previous
     section was particularly light on any code that could actually
     do the job and store the person's data to a database. There
     was no serialization or deserialization code, not even data member
     registration, that you would normally have to write by hand in
     other ORM libraries for C++. This is because in ODB code
     that translates between the database and C++ representations
     of an object is automatically generated by the ODB compiler.</p>

  <p>To compile the <code>person.hxx</code> header we created in the
     previous section and generate the support code for the MySQL
     database, we invoke the ODB compiler from a terminal (UNIX) or
     a command prompt (Windows):</p>

  <pre class="terminal">
odb -d mysql --generate-query person.hxx
  </pre>

  <p>We will use MySQL as the database of choice in the remainder of
     this chapter, though other supported database systems can be used
     instead.</p>

  <p>If you haven't installed the common ODB runtime library
     (<code>libodb</code>) or installed it into a directory where
     C++ compilers don't search for headers by default,
     then you may get the following error:</p>

  <pre class="terminal">
person.hxx:10:24: fatal error: odb/core.hxx: No such file or directory
  </pre>

  <p>To resolve this you will need to specify the <code>libodb</code> headers
     location with the <code>-I</code> preprocessor option, for example:</p>

  <pre class="terminal">
odb -I.../libodb -d mysql --generate-query person.hxx
  </pre>

  <p>Here <code>.../libodb</code> represents the path to the
     <code>libodb</code> directory.</p>

  <p>The above invocation of the ODB compiler produces three C++ files:
     <code>person-odb.hxx</code>, <code>person-odb.ixx</code>,
     <code>person-odb.cxx</code>. You normally don't use types
     or functions contained in these files directly. Rather, all
     you have to do is include <code>person-odb.hxx</code> in
     C++ files where you are performing database operations
     with classes from <code>person.hxx</code> as well as compile
     <code>person-odb.cxx</code> and link the resulting object
     file to your application.</p>

  <p>You may be wondering what the <code>--generate-query</code>
     option is for. It instructs the ODB compiler to generate
     optional query support code that we will use later in our
     "Hello World" example. Another option that we will find
     useful is <code>--generate-schema</code>. This option
     makes the ODB compiler generate a fourth file,
     <code>person.sql</code>, which is the database schema
     for the persistent classes defined in <code>person.hxx</code>:</p>

  <pre class="terminal">
odb -d mysql --generate-query --generate-schema person.hxx
  </pre>

  <p>The database schema file contains SQL statements that creates
     tables necessary to store the persistent classes. We will learn
     how to use it in the next section.</p>

  <p>If you would like to see a list of all the available ODB compiler
     options, refer to the
     <a href="http://www.codesynthesis.com/products/odb/doc/odb.xhtml">ODB
     Compiler Command Line Manual</a>.</p>

  <p>Now that we have the persistent class and the database support
     code, the only part that is left is the application code that
     does something useful with all of this. But before we move on to
     the fun part, let's first learn how to build and run an application
     that uses ODB. This way when we have some application code
     to try, there are no more delays before we can run it.</p>

  <h2><a name="2.3">2.3 Compiling and Running</a></h2>

  <p>Assuming that the <code>main()</code> function with the application
     code is saved in <code>driver.cxx</code> and the database support
     code and schema are generated as described in the previous section,
     to build our application we will first need to compile all the C++
     source files and then link them with two ODB runtime libraries.</p>

  <p>On UNIX, the compilation part can be done with the following commands
     (substitute <code>c++</code> with your C++ compiler name; for Microsoft
     Visual Studio setup, see the <code>odb-examples</code> package):</p>

  <pre class="terminal">
c++ -c driver.cxx
c++ -c person-odb.cxx
  </pre>

  <p>Similar to the ODB compilation, if you get an error stating that
  a header in <code>odb/</code> or <code>odb/mysql</code> directory
  is not found, you will need to use the <code>-I</code>
  preprocessor option to specify the location of the common ODB runtime
  library (<code>libodb</code>) and MySQL ODB runtime library
  (<code>libodb-mysql</code>).</p>

  <p>Once the compilation is done, we can link the application with
  the following command:</p>

  <pre class="terminal">
c++ -o driver driver.o person-odb.o -lodb-mysql -lodb
  </pre>

  <p>Notice that we link our application with two ODB libraries:
    <code>libodb</code> which is a common runtime library and
    <code>libodb-mysql</code> which is a MySQL runtime library
    (if you use another database, then the name of this library
    will change accordingly). If you get an error saying that
    one of these libraries could not be found, then you will need
    to use the <code>-L</code> linker option to specify their locations.</p>

  <p>Before we can run our application we need to create a database
    schema using the generated <code>person.sql</code> file. For MySQL
    we can use the <code>mysql</code> client program, for example:</p>

  <pre class="terminal">
mysql --user=odb_test --database=odb_test &lt; person.sql
  </pre>

  <p>The above command will log in to a local MySQL server as user
    <code>odb_test</code> without a password and use the database
    named <code>odb_test</code>. Beware that after executing this
    command, all the data stored in the <code>odb_test</code> database
    will be deleted.</p>

  <p>Note also that using a standalone generated SQL file is not the
     only way to create a database schema in ODB. We can also embed
     the schema directly into our application or use custom schemas
     that were not generated by the ODB compiler. Refer to
     <a href="#3.4">Section 3.4, "Database"</a> for details.</p>

  <p>Once the database schema is ready, we run our application
  using the same login and database name:</p>

  <pre class="terminal">
./driver --user odb_test --database odb_test
  </pre>


  <h2><a name="2.4">2.4 Making Objects Persistent</a></h2>

  <p>Now that we have the infrastructure work out of the way, it
  is time to see our first code fragment that interacts with the
  database. In this section we will learn how to make <code>person</code>
  objects persistent:</p>

  <pre class="cxx">
// driver.cxx
//

#include &lt;memory>   // std::unique_ptr
#include &lt;iostream>

#include &lt;odb/database.hxx>
#include &lt;odb/transaction.hxx>

#include &lt;odb/mysql/database.hxx>

#include "person.hxx"
#include "person-odb.hxx"

using namespace std;
using namespace odb::core;

int
main (int argc, char* argv[])
{
  try
  {
    unique_ptr&lt;database> db (new odb::mysql::database (argc, argv));

    unsigned long long john_id, jane_id, joe_id;

    // Create a few persistent person objects.
    //
    {
      person john ("John", "Doe", 33);
      person jane ("Jane", "Doe", 32);
      person joe ("Joe", "Dirt", 30);

      transaction t (db->begin ());

      // Make objects persistent and save their ids for later use.
      //
      john_id = db->persist (john);
      jane_id = db->persist (jane);
      joe_id = db->persist (joe);

      t.commit ();
    }
  }
  catch (const odb::exception&amp; e)
  {
    cerr &lt;&lt; e.what () &lt;&lt; endl;
    return 1;
  }
}
  </pre>

  <p>Let's examine this code piece by piece. At the beginning we include
     a bunch of headers. After the standard C++ headers we include
     <code>&lt;odb/database.hxx></code>
     and <code>&lt;odb/transaction.hxx></code> which define database
     system-independent <code>odb::database</code> and
     <code>odb::transaction</code> interfaces. Then we include
     <code>&lt;odb/mysql/database.hxx></code> which defines the
     MySQL implementation of the <code>database</code> interface. Finally,
     we include <code>person.hxx</code> and <code>person-odb.hxx</code>
     which define our persistent <code>person</code> class.</p>

  <p>Then we have two <code>using namespace</code> directives. The first
     one brings in the names from the standard namespace and the second
     brings in the ODB declarations which we will use later in the file.
     Notice that in the second directive we use the <code>odb::core</code>
     namespace instead of just <code>odb</code>. The former only brings
     into the current namespace the essential ODB names, such as the
     <code>database</code> and <code>transaction</code> classes, without
     any of the auxiliary objects. This minimizes the likelihood of name
     conflicts with other libraries. Note also that you should continue
     using the <code>odb</code> namespace when qualifying individual names.
     For example, you should write <code>odb::database</code>, not
     <code>odb::core::database</code>.</p>

  <p>Once we are in <code>main()</code>, the first thing we do is create
     the MySQL database object. Notice that this is the last line in
     <code>driver.cxx</code> that mentions MySQL explicitly; the rest
     of the code works through the common interfaces and is database
     system-independent. We use the <code>argc</code>/<code>argv</code>
     <code>mysql::database</code> constructor which automatically
     extract the database parameters, such as login name, password,
     database name, etc., from the command line. In your own applications
     you may prefer to use other <code>mysql::database</code>
     constructors which allow you to pass this information directly
     (<a href="#17.2">Section 17.2, "MySQL Database Class"</a>).</p>

  <p>Next, we create three <code>person</code> objects. Right now they are
     transient objects, which means that if we terminate the application
     at this point, they will be gone without any evidence of them ever
     existing. The next line starts a database transaction. We discuss
     transactions in detail later in this manual. For now, all we need
     to know is that all ODB database operations must be performed within
     a transaction and that a transaction is an atomic unit of work; all
     database operations performed within a transaction either succeed
     (committed) together or are automatically undone (rolled back).</p>

  <p>Once we are in a transaction, we call the <code>persist()</code>
     database function on each of our <code>person</code> objects.
     At this point the state of each object is saved in the database.
     However, note that this state is not permanent until and unless
     the transaction is committed. If, for example, our application
     crashes at this point, there will still be no evidence of our
     objects ever existing.</p>

  <p>In our case, one more thing happens when we call <code>persist()</code>.
     Remember that we decided to use database-assigned identifiers for our
     <code>person</code> objects. The call to <code>persist()</code> is
     where this assignment happens. Once this function returns, the
     <code>id_</code> member contains this object's unique identifier.
     As a convenience, the <code>persist()</code> function also returns
     a copy of the object's identifier that it made persistent. We
     save the returned identifier for each object in a local variable.
     We will use these identifiers later in the chapter to perform other
     database operations on our persistent objects.</p>

  <p>After we have persisted our objects, it is time to commit the
     transaction and make the changes permanent. Only after the
     <code>commit()</code> function returns successfully, are we
     guaranteed that the objects are made persistent. Continuing
     with the crash example, if our application terminates after
     the commit for whatever reason, the objects' state in the
     database will remain intact. In fact, as we will discover
     shortly, our application can be restarted and load the
     original objects from the database. Note also that a
     transaction must be committed explicitly with the
     <code>commit()</code> call. If the <code>transaction</code>
     object leaves scope without the transaction being
     explicitly committed or rolled back, it will automatically be
     rolled back. This behavior allows you not to worry about
     exceptions being thrown within a transaction; if they
     cross the transaction boundary, the transaction will
     automatically be rolled back and all the changes made
     to the database undone.</p>

  <p>The final bit of code in our example is the <code>catch</code>
     block that handles the database exceptions. We do this by catching
     the base ODB exception (<a href="#3.14">Section 3.14, "ODB
     Exceptions"</a>) and printing the diagnostics.</p>

  <p>Let's now compile (<a href="#2.3">Section 2.3, "Compiling and
     Running"</a>) and then run our first ODB application:</p>

  <pre class="terminal">
mysql --user=odb_test --database=odb_test &lt; person.sql
./driver --user odb_test --database odb_test
  </pre>

  <p>Our first application doesn't print anything except for error
     messages so we can't really tell whether it actually stored the
     objects' state in the database. While we will make our application
     more entertaining shortly, for now we can use the <code>mysql</code>
     client to examine the database content. It will also give us a feel
     for how the objects are stored:</p>

  <pre class="terminal">
mysql --user=odb_test --database=odb_test

Welcome to the MySQL monitor.

mysql> select * from person;

+----+-------+------+-----+
| id | first | last | age |
+----+-------+------+-----+
|  1 | John  | Doe  |  33 |
|  2 | Jane  | Doe  |  32 |
|  3 | Joe   | Dirt |  30 |
+----+-------+------+-----+
3 rows in set (0.00 sec)

mysql> quit
  </pre>

  <p>Another way to get more insight into what's going on under the hood,
     is to trace the SQL statements executed by ODB as a result of
     each database operation. Here is how we can enable tracing just for
     the duration of our transaction:</p>

  <pre class="cxx">
    // Create a few persistent person objects.
    //
    {
      ...

      transaction t (db->begin ());

      t.tracer (stderr_tracer);

      // Make objects persistent and save their ids for later use.
      //
      john_id = db->persist (john);
      jane_id = db->persist (jane);
      joe_id = db->persist (joe);

      t.commit ();
    }
  </pre>

  <p>With this modification our application now produces the following
     output:</p>

  <pre class="terminal">
INSERT INTO `person` (`id`,`first`,`last`,`age`) VALUES (?,?,?,?)
INSERT INTO `person` (`id`,`first`,`last`,`age`) VALUES (?,?,?,?)
INSERT INTO `person` (`id`,`first`,`last`,`age`) VALUES (?,?,?,?)
  </pre>

  <p>Note that we see question marks instead of the actual values
     because ODB uses prepared statements and sends the data to the
     database in binary form. For more information on tracing, refer
     to <a href="#3.13">Section 3.13, "Tracing SQL Statement Execution"</a>.
     In the next section we will see how to access persistent objects
     from our application.</p>

  <h2><a name="2.5">2.5 Querying the Database for Objects</a></h2>

  <p>So far our application doesn't resemble a typical "Hello World"
     example. It doesn't print anything except for error messages.
     Let's change that and teach our application to say hello to
     people from our database. To make it a bit more interesting,
     let's say hello only to people over 30:</p>

  <pre class="cxx">
// driver.cxx
//

...

int
main (int argc, char* argv[])
{
  try
  {
    ...

    // Create a few persistent person objects.
    //
    {
      ...
    }

    using query = odb::query&lt;person>;
    using result = odb::result&lt;person>;

    // Say hello to those over 30.
    //
    {
      transaction t (db->begin ());

      result r (db->query&lt;person> (query::age > 30));

      for (result::iterator i (r.begin ()); i != r.end (); ++i)
      {
        cout &lt;&lt; "Hello, " &lt;&lt; i->first () &lt;&lt; "!" &lt;&lt; endl;
      }

      t.commit ();
    }
  }
  catch (const odb::exception&amp; e)
  {
    cerr &lt;&lt; e.what () &lt;&lt; endl;
    return 1;
  }
}
  </pre>

  <p>The first half of our application is the same as before and is
     replaced with "..." in the above listing for brevity. Again, let's
     examine the rest of it piece by piece.</p>

  <p>The two <code>using</code> declarations create convenient aliases for two
     template instantiations that will be used a lot in our application. The
     first is the query type for the <code>person</code> objects and the
     second is the result type for that query.</p>

  <p>Then we begin a new transaction and call the <code>query()</code>
     database function. We pass a query expression
     (<code>query::age > 30</code>) which limits the returned objects
     only to those with the age greater than 30. We also save the result
     of the query in a local variable.</p>

  <p>The next few lines perform a standard for-loop iteration
     over the result sequence printing hello for every returned person.
     Then we commit the transaction and that's it. Let's see what
     this application will print:</p>

  <pre class="terminal">
mysql --user=odb_test --database=odb_test &lt; person.sql
./driver --user odb_test --database odb_test

Hello, John!
Hello, Jane!
  </pre>


  <p>That looks about right, but how do we know that the query actually
     used the database instead of just using some in-memory artifacts of
     the earlier <code>persist()</code> calls? One way to test this
     would be to comment out the first transaction in our application
     and re-run it without re-creating the database schema. This way the
     objects that were persisted during the previous run will be returned.
     Alternatively, we can just re-run the same application without
     re-creating the schema and notice that we now show duplicate
     objects:</p>

  <pre class="terminal">
./driver --user odb_test --database odb_test

Hello, John!
Hello, Jane!
Hello, John!
Hello, Jane!
  </pre>

  <p>What happens here is that the previous run of our application
     persisted a set of <code>person</code> objects and when we re-run
     the application, we persist another set with the same names but
     with different ids. When we later run the query, matches from
     both sets are returned. We can change the line where we print
     the "Hello" string as follows to illustrate this point:</p>

  <pre class="cxx">
cout &lt;&lt; "Hello, " &lt;&lt; i->first () &lt;&lt; " (" &lt;&lt; i->id () &lt;&lt; ")!" &lt;&lt; endl;
  </pre>

  <p>If we now re-run this modified program, again without re-creating
     the database schema, we will get the following output:</p>

  <pre class="terminal">
./driver --user odb_test --database odb_test

Hello, John (1)!
Hello, Jane (2)!
Hello, John (4)!
Hello, Jane (5)!
Hello, John (7)!
Hello, Jane (8)!
  </pre>

  <p>The identifiers 3, 6, and 9 that are missing from the above list belong
     to the "Joe Dirt" objects which are not selected by this query.</p>

  <h2><a name="2.6">2.6 Updating Persistent Objects</a></h2>

  <p>While making objects persistent and then selecting some of them using
     queries are two useful operations, most applications will also need
     to change the object's state and then make these changes persistent.
     Let's illustrate this by updating Joe's age who just had a birthday:</p>

  <pre class="cxx">
// driver.cxx
//

...

int
main (int argc, char* argv[])
{
  try
  {
    ...

    unsigned long long john_id, jane_id, joe_id;

    // Create a few persistent person objects.
    //
    {
      ...

      // Save object ids for later use.
      //
      john_id = john.id ();
      jane_id = jane.id ();
      joe_id = joe.id ();
    }

    // Joe Dirt just had a birthday, so update his age.
    //
    {
      transaction t (db->begin ());

      unique_ptr&lt;person> joe (db->load&lt;person> (joe_id));
      joe->age (joe->age () + 1);
      db->update (*joe);

      t.commit ();
    }

    // Say hello to those over 30.
    //
    {
      ...
    }
  }
  catch (const odb::exception&amp; e)
  {
    cerr &lt;&lt; e.what () &lt;&lt; endl;
    return 1;
  }
}
  </pre>

  <p>The beginning and the end of the new transaction are the same as
     the previous two. Once within a transaction, we call the
     <code>load()</code> database function to instantiate a
     <code>person</code> object with Joe's persistent state. We
     pass Joe's object identifier that we stored earlier when we
     made this object persistent. While here we use
     <code>std::unique_ptr</code> to manage the returned object, we
     could have also used another smart pointer, for example
     <code>shared_ptr</code> from C++11 or Boost. For more information on the
     object lifetime management and the smart pointers that we can use for
     that, see <a href="#3.3">Section 3.3, "Object and View Pointers"</a>.</p>

  <p>With the instantiated object in hand we increment the age
     and call the <code>update()</code> function to update
     the object's state in the database. Once the transaction is
     committed, the changes are made permanent.</p>

  <p>If we now run this application, we will see Joe in the output
     since he is now over 30:</p>

  <pre class="terminal">
mysql --user=odb_test --database=odb_test &lt; person.sql
./driver --user odb_test --database odb_test

Hello, John!
Hello, Jane!
Hello, Joe!
  </pre>

  <p>What if we didn't have an identifier for Joe? Maybe this object
     was made persistent in another run of our application or by another
     application altogether. Provided that we only have one Joe Dirt
     in the database, we can use the query facility to come up with
     an alternative implementation of the above transaction:</p>

  <pre class="cxx">
    // Joe Dirt just had a birthday, so update his age. An
    // alternative implementation without using the object id.
    //
    {
      transaction t (db->begin ());

      // Here we know that there can be only one Joe Dirt in our
      // database so we use the query_one() shortcut instead of
      // manually iterating over the result returned by query().
      //
      unique_ptr&lt;person> joe (
        db->query_one&lt;person> (query::first == "Joe" &amp;&amp;
                               query::last == "Dirt"));

      if (joe.get () != 0)
      {
        joe->age (joe->age () + 1);
        db->update (*joe);
      }

      t.commit ();
    }
  </pre>

  <h2><a name="2.7">2.7 Defining and Using Views</a></h2>

  <p>Suppose that we need to gather some basic statistics about the people
     stored in our database. Things like the total head count, as well as
     the minimum and maximum ages. One way to do it would be to query
     the database for all the <code>person</code> objects and then
     calculate this information as we iterate over the query result.
     While this approach may work fine for our database with just three
     people in it, it would be very inefficient if we had a large
     number of objects.</p>

  <p>While it may not be conceptually pure from the object-oriented
     programming point of view, a relational database can perform
     some computations much faster and much more economically than
     if we performed the same operations ourselves in the application's
     process.</p>

  <p>To support such cases ODB provides the notion of views. An ODB view
     is a C++ <code>class</code> that embodies a light-weight, read-only
     projection of one or more persistent objects or database tables or
     the result of a native SQL query execution or stored procedure
     call.</p>

  <p>Some of the common applications of views include loading a subset of
     data members from objects or columns database tables, executing and
     handling results of arbitrary SQL queries, including aggregate
     queries, as well as joining multiple objects and/or database
     tables using object relationships or custom join conditions.</p>

  <p>While you can find a much more detailed description of views in
     <a href="#10">Chapter 10, "Views"</a>, here is how we can define
     the <code>person_stat</code> view that returns the basic statistics
     about the <code>person</code> objects:</p>

  <pre class="cxx">
#pragma db view object(person)
struct person_stat
{
  #pragma db column("count(" + person::id_ + ")")
  std::size_t count;

  #pragma db column("min(" + person::age_ + ")")
  unsigned short min_age;

  #pragma db column("max(" + person::age_ + ")")
  unsigned short max_age;
};
  </pre>

  <p>Normally, to get the result of a view we use the same
     <code>query()</code> function as when querying the database for
     an object. Here, however, we are executing an aggregate query
     which always returns exactly one element. Therefore, instead
     of getting the result instance and then iterating over it, we
     can use the shortcut <code>query_value()</code> function. Here is
     how we can load and print our statistics using the view we have
     just created:</p>

  <pre class="cxx">
    // Print some statistics about all the people in our database.
    //
    {
      transaction t (db->begin ());

      // The result of this query always has exactly one element.
      //
      person_stat ps (db->query_value&lt;person_stat> ());

      cout &lt;&lt; "count  : " &lt;&lt; ps.count &lt;&lt; endl
           &lt;&lt; "min age: " &lt;&lt; ps.min_age &lt;&lt; endl
           &lt;&lt; "max age: " &lt;&lt; ps.max_age &lt;&lt; endl;

      t.commit ();
    }
  </pre>

  <p>If we now add the <code>person_stat</code> view to the
     <code>person.hxx</code> header, the above transaction
     to <code>driver.cxx</code>, as well as re-compile and
     re-run our example, then we will see the following
     additional lines in the output:</p>

  <pre class="term">
count  : 3
min age: 31
max age: 33
  </pre>

  <h2><a name="2.8">2.8 Deleting Persistent Objects</a></h2>

  <p>The last operation that we will discuss in this chapter is deleting
     the persistent object from the database. The following code
     fragment shows how we can delete an object given its identifier:</p>

  <pre class="cxx">
    // John Doe is no longer in our database.
    //
    {
      transaction t (db->begin ());
      db->erase&lt;person> (john_id);
      t.commit ();
    }
  </pre>

  <p>To delete John from the database we start a transaction, call
     the <code>erase()</code> database function with John's object
     id, and commit the transaction. After the transaction is committed,
     the erased object is no longer persistent.</p>

  <p>If we don't have an object id handy, we can use queries to find
     and delete the object:</p>

  <pre class="cxx">
    // John Doe is no longer in our database. An alternative
    // implementation without using the object id.
    //
    {
      transaction t (db->begin ());

      // Here we know that there can be only one John Doe in our
      // database so we use the query_one() shortcut again.
      //
      unique_ptr&lt;person> john (
        db->query_one&lt;person> (query::first == "John" &amp;&amp;
                               query::last == "Doe"));

      if (john.get () != 0)
        db->erase (*john);

      t.commit ();
    }
  </pre>

  <h2><a name="2.9">2.9 Changing Persistent Classes</a></h2>

  <p>When the definition of a transient C++ class is changed, for
     example by adding or deleting a data member, we don't have to
     worry about any existing instances of this class not matching
     the new definition. After all, to make the class changes
     effective we have to restart the application and none of the
     transient instances will survive this.</p>

  <p>Things are not as simple for persistent classes. Because they
     are stored in the database and therefore survive application
     restarts, we have a new problem: what happens to the state of
     existing objects (which correspond to the old definition) once
     we change our persistent class?</p>

  <p>The problem of working with old objects, called <em>database
     schema evolution</em>, is a complex issue and ODB provides
     comprehensive support for handling it. While this support
     is covered in detail in <a href="#13">Chapter 13,
     "Database Schema Evolution"</a>, let us consider a simple
     example that should give us a sense of the functionality
     provided by ODB in this area.</p>

  <p>Suppose that after using our <code>person</code> persistent
     class for some time and creating a number of databases
     containing its instances, we realized that for some people
     we also need to store their middle name. If we go ahead and
     just add the new data member, everything will work fine
     with new databases. Existing databases, however, have a
     table that does not correspond to the new class definition.
     Specifically, the generated database support code now
     expects there to be a column to store the middle name.
     But such a column was never created in the old databases.</p>

  <p>ODB can automatically generate SQL statements that will
     migrate old databases to match the new class definitions.
     But first, we need to enable schema evolution support by
     defining a version for our object model:</p>

  <pre class="cxx">
// person.hxx
//

#pragma db model version(1, 1)

class person
{
  ...

  std::string first_;
  std::string last_;
  unsigned short age_;
};
  </pre>

  <p>The first number in the <code>version</code> pragma is the
     base model version. This is the lowest version we will be
     able to migrate from. The second number is the current model
     version. Since we haven't made any changes yet to our
     persistent class, both of these values are <code>1</code>.</p>

  <p>Next we need to re-compile our <code>person.hxx</code> header
     file with the ODB compiler, just as we did before:</p>

  <pre class="terminal">
odb -d mysql --generate-query --generate-schema person.hxx
  </pre>

  <p>If we now look at the list of files produced by the ODB compiler,
     we will notice a new file: <code>person.xml</code>. This
     is a changelog file where the ODB compiler keeps track of the
     database changes corresponding to our class changes. Note that
     this file is automatically maintained by the ODB compiler and
     all we have to do is keep it around between re-compilations.</p>

  <p>Now we are ready to add the middle name to our <code>person</code>
     class. We also give it a default value (empty string) which
     is what will be assigned to existing objects in old databases.
     Notice that we have also incremented the current version:</p>

  <pre class="cxx">
// person.hxx
//

#pragma db model version(1, 2)

class person
{
  ...

  std::string first_;

  #pragma db default("")
  std::string middle_;

  std::string last_;
  unsigned short age_;
};
  </pre>

  <p>If we now recompile the <code>person.hxx</code> header again, we will
     see two extra generated files: <code>person-002-pre.sql</code>
     and <code>person-002-post.sql</code>. These two files contain
     schema migration statements from version <code>1</code> to
     version <code>2</code>. Similar to schema creation, schema
     migration statements can also be embedded into the generated
     C++ code.</p>

  <p><code>person-002-pre.sql</code> and <code>person-002-post.sql</code>
     are the pre and post schema migration files. To migrate
     one of our old databases, we first execute the pre migration
     file:</p>

  <pre class="terminal">
mysql --user=odb_test --database=odb_test &lt; person-002-pre.sql
  </pre>

  <p>Between the pre and post schema migrations we can run data
     migration code, if required. At this stage, we can both
     access the old and store the new data. In our case we don't
     need any data migration code since we assigned the default
     value to the middle name for all the existing objects.</p>

  <p>To finish the migration process we execute the post migration
     statements:</p>

  <pre class="terminal">
mysql --user=odb_test --database=odb_test &lt; person-002-post.sql
  </pre>

  <h2><a name="2.10">2.10 Working with Multiple Databases</a></h2>

  <p>Accessing multiple databases (that is, data stores) is simply a
     matter of creating multiple <code>odb::&lt;db>::database</code>
     instances representing each database. For example:</p>

  <pre class="cxx">
odb::mysql::database db1 ("john", "secret", "test_db1");
odb::mysql::database db2 ("john", "secret", "test_db2");
  </pre>

  <p>Some database systems also allow attaching multiple databases to
     the same instance. A more interesting question is how we access
     multiple database systems (that is, database implementations) from
     the same application. For example, our application may need to store
     some objects in a remote MySQL database and others in a local SQLite
     file. Or, our application may need to be able to store its objects
     in a database system that is selected by the user at runtime.</p>

  <p>ODB provides comprehensive multi-database support that ranges from
     tight integration with specific database systems to being able to
     write database-agnostic code and loading individual database systems
     support dynamically. While all these aspects are covered in detail
     in <a href="#16">Chapter 16, "Multi-Database Support"</a>, in this
     section we will get a taste of this functionality by extending our
     "Hello World" example to be able to store its data either in MySQL
     or PostgreSQL (other database systems supported by ODB can be added
     in a similar manner).</p>

  <p>The first step in adding multi-database support is to re-compile
     our <code>person.hxx</code> header to generate database support
     code for additional database systems:</p>

  <pre class="terminal">
odb --multi-database dynamic -d common -d mysql -d pgsql \
--generate-query --generate-schema person.hxx
  </pre>

  <p>The <code>--multi-database</code> ODB compiler option turns on
     multi-database support. For now it is not important what the
     <code>dynamic</code> value that we passed to this option means, but
     if you are curious, see <a href="#16">Chapter 16</a>. The result of this
     command are three sets of generated files: <code>person-odb.?xx</code>
     (common interface; corresponds to the <code>common</code> database),
     <code>person-odb-mysql.?xx</code> (MySQL support code), and
     <code>person-odb-pgsql.?xx</code> (PostgreSQL support code). There
     are also two schema files: <code>person-mysql.sql</code> and
     <code>person-pgsql.sql</code>.</p>

  <p>The only part that we need to change in <code>driver.cxx</code>
     is how we create the database instance. Specifically, this line:</p>

  <pre class="cxx">
unique_ptr&lt;database> db (new odb::mysql::database (argc, argv));
  </pre>

  <p>Now our example is capable of storing its data either in MySQL or
     PostgreSQL so we need to somehow allow the caller to specify which
     database we must use. To keep things simple, we will make the first
     command line argument specify the database system we must use while
     the rest will contain the database-specific options which we will
     pass to the <code>odb::&lt;db>::database</code> constructor as
     before. Let's put all this logic into a separate function which we
     will call <code>create_database()</code>. Here is what the beginning
     of our modified <code>driver.cxx</code> will look like (the remainder
     is unchanged):</p>

  <pre class="cxx">
// driver.cxx
//

#include &lt;string>
#include &lt;memory>   // std::unique_ptr
#include &lt;iostream>

#include &lt;odb/database.hxx>
#include &lt;odb/transaction.hxx>

#include &lt;odb/mysql/database.hxx>
#include &lt;odb/pgsql/database.hxx>

#include "person.hxx"
#include "person-odb.hxx"

using namespace std;
using namespace odb::core;

unique_ptr&lt;database>
create_database (int argc, char* argv[])
{
  unique_ptr&lt;database> r;

  if (argc &lt; 2)
  {
    cerr &lt;&lt; "error: database system name expected" &lt;&lt; endl;
    return r;
  }

  string db (argv[1]);

  if (db == "mysql")
    r.reset (new odb::mysql::database (argc, argv));
  else if (db == "pgsql")
    r.reset (new odb::pgsql::database (argc, argv));
  else
    cerr &lt;&lt; "error: unknown database system " &lt;&lt; db &lt;&lt; endl;

  return r;
}

int
main (int argc, char* argv[])
{
  try
  {
    unique_ptr&lt;database> db (create_database (argc, argv));

    if (db.get () == 0)
      return 1; // Diagnostics has already been issued.

    ...
  </pre>

  <p>And that's it. The only thing left is to build and run our
     example:</p>

  <pre class="terminal">
c++ -c driver.cxx
c++ -c person-odb.cxx
c++ -c person-odb-mysql.cxx
c++ -c person-odb-pgsql.cxx
c++ -o driver driver.o person-odb.o person-odb-mysql.o \
person-odb-pgsql.o -lodb-mysql -lodb-pgsql -lodb
  </pre>

  <p>Here is how we can access a MySQL database:</p>

  <pre class="terminal">
mysql --user=odb_test --database=odb_test &lt; person-mysql.sql
./driver mysql --user odb_test --database odb_test
  </pre>

  <p>Or a PostgreSQL database:</p>

  <pre class="terminal">
psql --user=odb_test --dbname=odb_test -f person-pgsql.sql
./driver pgsql --user odb_test --database odb_test
  </pre>

  <h2><a name="2.11">2.11 Summary</a></h2>

  <p>This chapter presented a very simple application which, nevertheless,
     exercised all of the core database functions: <code>persist()</code>,
     <code>query()</code>, <code>load()</code>, <code>update()</code>,
     and <code>erase()</code>. We also saw that writing an application
     that uses ODB involves the following steps:</p>

  <ol>
    <li>Declare persistent classes in header files.</li>
    <li>Compile these headers to generate database support code.</li>
    <li>Link the application with the generated code and two ODB runtime
        libraries.</li>
  </ol>

  <p>Do not be concerned if, at this point, much appears unclear. The intent
     of this chapter is to give you only a general idea of how to persist C++
     objects with ODB. We will cover all the details throughout the remainder
     of this manual.</p>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="3">3 Working with Persistent Objects</a></h1>

  <p>The previous chapters gave us a high-level overview of ODB and
     showed how to use it to store C++ objects in a database. In this
     chapter we will examine the ODB object persistence model as
     well as the core database APIs in greater detail. We will
     start with basic concepts and terminology in <a href="#3.1">Section
     3.1</a> and <a href="#3.3">Section 3.3</a> and continue with the
     discussion of the <code>odb::database</code> class in
     <a href="#3.4">Section 3.4</a>, transactions in
     <a href="#3.5">Section 3.5</a>, and connections in
     <a href="#3.6">Section 3.6</a>. The remainder of this chapter
     deals with the core database operations and concludes with
     the discussion of ODB exceptions.</p>

  <p>In this chapter we will continue to use and expand the
     <code>person</code> persistent class that we have developed in the
     previous chapter.</p>

  <h2><a name="3.1">3.1 Concepts and Terminology</a></h2>

  <p>The term <em>database</em> can refer to three distinct things:
     a general notion of a place where an application stores its data,
     a software implementation for managing this data (for example
     MySQL), and, finally, some database software implementations
     may manage several data stores which are usually distinguished
     by name. This name is also commonly referred to as a database.</p>

  <p>In this manual, when we use the word <em>database</em>, we
     refer to the first meaning above, for example,
     "The <code>update()</code> function saves the object's state to
     the database." The term Database Management System (DBMS) is
     often used to refer to the second meaning of the word database.
     In this manual we will use the term <em>database system</em>
     for short, for example, "Database system-independent
     application code." Finally, to distinguish the third meaning
     from the other two, we will use the term <em>database name</em>,
     for example, "The second option specifies the database name
     that the application should use to store its data."</p>

  <p>In C++ there is only one notion of a type and an instance
     of a type. For example, a fundamental type, such as <code>int</code>,
     is, for the most part, treated the same as a user defined class
     type. However, when it comes to persistence, we have to place
     certain restrictions and requirements on certain C++ types that
     can be stored in the database. As a result, we divide persistent
     C++ types into two groups: <em>object types</em> and <em>value
     types</em>. An instance of an object type is called an <em>object</em>
     and an instance of a value type &mdash; a <em>value</em>.</p>

  <p>An object is an independent entity. It can be stored, updated,
     and deleted in the database independent of other objects.
     Normally, an object has an identifier, called <em>object id</em>,
     that is unique among all instances of an object type within a
     database. In contrast, a value can only be stored in the database
     as part of an object and doesn't have its own unique identifier.</p>

  <p>An object consists of data members which are either values
     (<a href="#7">Chapter 7, "Value Types"</a>), pointers
     to other objects (<a href="#6">Chapter 6, "Relationships"</a>), or
     containers of values or pointers to other objects (<a href="#5">Chapter
     5, "Containers")</a>. Pointers to other objects and containers can
     be viewed as special kinds of values since they also can only
     be stored in the database as part of an object. Static data members
     are not stored in the database.</p>

  <p>An object type is a C++ class. Because of this one-to-one
     relationship, we will use terms <em>object type</em>
     and <em>object class</em> interchangeably. In contrast,
     a value type can be a fundamental C++ type, such as
     <code>int</code> or a class type, such as <code>std::string</code>.
     If a value consists of other values, then it is called a
     <em>composite value</em> and its type &mdash; a
     <em>composite value type</em> (<a href="#7.2">Section 7.2,
     "Composite Value Types"</a>). Otherwise, the value is
     called <em>simple value</em> and its type &mdash; a
     <em>simple value type</em> (<a href="#7.1">Section 7.1,
     "Simple Value Types"</a>). Note that the distinction between
     simple and composite values is conceptual rather than
     representational. For example, <code>std::string</code>
     is a simple value type because conceptually string is a
     single value even though the representation of the string
     class may contain several data members each of which could be
     considered a value. In fact, the same value type can be
     viewed (and mapped) as both simple and composite by different
     applications.</p>

  <p>While not strictly necessary in a purely object-oriented application,
     practical considerations often require us to only load a
     subset of an object's data members or a combination of members
     from several objects. We may also need to factor out some
     computations to the relational database instead of performing
     them in the application's process. To support such requirements
     ODB distinguishes a third kind of C++ types, called <em>views</em>
     (<a href="#10">Chapter 10, "Views"</a>). An ODB view is a C++
     <code>class</code> that embodies a light-weight, read-only
     projection of one or more persistent objects or database
     tables or the result of a native SQL query execution.</p>

  <p>Understanding how all these concepts map to the relational model
     will hopefully make these distinctions clearer. In a relational
     database an object type is mapped to a table and a value type is
     mapped to one or more columns. A simple value type is mapped
     to a single column while a composite value type is mapped to
     several columns. An object is stored as a row in this
     table and a value is stored as one or more cells in this row.
     A simple value is stored in a single cell while a composite
     value occupies several cells. A view is not a persistent
     entity and it is not stored in the database. Rather, it is a
     data structure that is used to capture a single row of an SQL
     query result.</p>

  <p>Going back to the distinction between simple and composite
     values, consider a date type which has three integer
     members: year, month, and day. In one application it can be
     considered a composite value and each member will get its
     own column in a relational database. In another application
     it can be considered a simple value and stored in a single
     column as a number of days from some predefined date.</p>

  <p>Until now, we have been using the term <em>persistent class</em>
     to refer to object classes. We will continue to do so even though
     a value type can also be a class. The reason for this asymmetry
     is the subordinate nature of value types when it comes to
     database operations. Remember that values are never stored
     directly but rather as part of an object that contains them.
     As a result, when we say that we want to make a C++ class
     persistent or persist an instance of a class in the database,
     we invariably refer to an object class rather than a value
     class.</p>

  <p>Normally, you would use object types to model real-world entities,
     things that have their own identity. For example, in the
     previous chapter we created a <code>person</code> class to model
     a person, which is a real-world entity. Name and age, which we
     used as data members in our <code>person</code> class are clearly
     values. It is hard to think of age 31 or name "Joe" as having their
     own identities.</p>

  <p>A good test to determine whether something is an object or
     a value, is to consider if other objects might reference
     it. A person is clearly an object because it can be referred
     to by other objects such as a spouse, an employer, or a
     bank. On the other hand, a person's age or name is not
     something that other objects would normally refer to.</p>

  <p>Also, when an object represents a real entity, it is easy to
     choose a suitable object id. For example, for a
     person there is an established notion of an identifier
     (SSN, student id, passport number, etc). Another alternative
     is to use a person's email address as an identifier.</p>

  <p>Note, however, that these are only guidelines. There could
     be good reasons to make something that would normally be
     a value an object. Consider, for example, a database that
     stores a vast number of people. Many of the <code>person</code>
     objects in this database have the same names and surnames and
     the overhead of storing them in every object may negatively
     affect the performance. In this case, we could make the first name
     and last name each an object and only store pointers to
     these objects in the <code>person</code> class.</p>

  <p>An instance of a persistent class can be in one of two states:
    <em>transient</em> and <em>persistent</em>. A transient
    instance only has a representation in the application's
    memory and will cease to exist when the application terminates,
    unless it is explicitly made persistent. In other words, a
    transient instance of a persistent class behaves just like an
    instance of any ordinary C++ class. A persistent instance
    has a representation in both the application's memory and the
    database. A persistent instance will remain even after the
    application terminates unless and until it is explicitly
    deleted from the database.</p>

  <h2><a name="3.2">3.2 Declaring Persistent Objects and Values</a></h2>

  <p>To make a C++ class a persistent object class we declare
     it as such using the <code>db&nbsp;object</code> pragma, for
     example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
};
  </pre>

  <p>The other pragma that we often use is <code>db&nbsp;id</code>
     which designates one of the data members as an object id, for
     example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db id
  unsigned long long id_;
};
  </pre>

  <p>The object id can be of a simple or composite (<a href="#7.2.1">Section
     7.2.1, "Composite Object Ids"</a>) value type. This type should be
     default-constructible, copy-constructible, and copy-assignable. It
     is also possible to declare a persistent class without an object id,
     however, such a class will have limited functionality
     (<a href="#14.1.6">Section 14.1.6, "<code>no_id</code>"</a>).</p>

  <p>The above two pragmas are the minimum required to declare a
     persistent class with an object id. Other pragmas can be used to
     fine-tune the database-related properties of a class and its
     members (<a href="#14">Chapter 14, "ODB Pragma Language"</a>).</p>

  <p>Normally, a persistent class should define the default constructor. The
     generated database support code uses this constructor when
     instantiating an object from the persistent state. If we add the
     default constructor only for the database support code, then we
     can make it private provided we also make the <code>odb::access</code>
     class, defined in the <code>&lt;odb/core.hxx></code> header, a
     friend of this object class. For example:</p>

  <pre class="cxx">
#include &lt;odb/core.hxx>

#pragma db object
class person
{
  ...

private:
  friend class odb::access;
  person () {}
};
  </pre>

  <p>It is also possible to have an object class without the default
     constructor. However, in this case, the database operations will
     only be able to load the persistent state into an existing instance
     (<a href="#3.9">Section 3.9, "Loading Persistent Objects"</a>,
     <a href="#4.4">Section 4.4, "Query Result"</a>).</p>

  <p>The ODB compiler also needs access to the non-transient
     (<a href="#14.4.11">Section 14.4.11, "<code>transient</code>"</a>)
     data members of a persistent class. The ODB compiler can access
     such data members directly if they are public. It can also do
     so if they are private or protected and the <code>odb::access</code>
     class is declared a friend of the object type. For example:</p>

  <pre class="cxx">
#include &lt;odb/core.hxx>

#pragma db object
class person
{
  ...

private:
  friend class odb::access;
  person () {}

  #pragma db id
  unsigned long long id_;

  std::string name_;
};
  </pre>

  <p>If data members are not accessible directly, then the ODB
     compiler will try to automatically find suitable accessor and
     modifier functions. To accomplish this, the ODB compiler will
     try to lookup common accessor and modifier names derived from
     the data member name. Specifically, for the <code>name_</code>
     data member in the above example, the ODB compiler will look
     for accessor functions with names: <code>get_name()</code>,
     <code>getName()</code>, <code>getname()</code>, and just
     <code>name()</code> as well as for modifier functions with
     names: <code>set_name()</code>, <code>setName()</code>,
     <code>setname()</code>, and just <code>name()</code>. You can
     also add support for custom name derivations with the
     <code>--accessor-regex</code> and <code>--modifier-regex</code>
     ODB compiler options. Refer to the
     <a href="http://www.codesynthesis.com/products/odb/doc/odb.xhtml">ODB
     Compiler Command Line Manual</a> for details on these options.
     The following example illustrates automatic accessor and modifier
     discovery:</p>

  <pre class="cxx">
#pragma db object
class person
{
public:
  person () {}

  ...

  unsigned long long id () const;
  void id (unsigned long long);

  const std::string&amp; get_name () const;
  std::string&amp; set_name ();

private:
  #pragma db id
  unsigned long long id_; // Uses id() for access.

  std::string name_; // Uses get_name()/set_name() for access.
};
  </pre>

  <p>Finally, if a data member is not directly accessible and the
     ODB compiler was unable to discover suitable accessor and
     modifier functions, then we can provide custom accessor
     and modifier expressions using the <code>db&nbsp;get</code>
     and <code>db&nbsp;set</code> pragmas. For more information
     on custom accessor and modifier expressions refer to
     <a href="#14.4.5">Section 14.4.5,
     "<code>get</code>/<code>set</code>/<code>access</code>"</a>.</p>

  <p>Data members of a persistent class can also be split into
     separately-loaded and/or separately-updated sections.
     For more information on this functionality, refer to
     <a href="#9">Chapter 9, "Sections"</a>.</p>

  <p>You may be wondering whether we also have to declare value types
     as persistent. We don't need to do anything special for simple value
     types such as <code>int</code> or <code>std::string</code> since the
     ODB compiler knows how to map them to suitable database types and
     how to convert between the two. On the other hand, if a simple value
     is unknown to the ODB compiler then we will need to provide the
     mapping to the database type and, possibly, the code to
     convert between the two. For more information on how to achieve
     this refer to the <code>db&nbsp;type</code> pragma description
     in <a href="#14.3.1">Section 14.3.1, "<code>type</code>"</a>.</p>

  <p>Similar to object classes, composite value types have to be
     explicitly declared as persistent using the <code>db&nbsp;value</code>
     pragma, for example:</p>

  <pre class="cxx">
#pragma db value
class name
{
  ...

  std::string first_;
  std::string last_;
};
  </pre>

  <p>Note that a composite value cannot have a data member designated
     as an object id since, as we have discussed earlier, values do
     not have a notion of identity. A composite value type also doesn't
     have to define the default constructor, unless it is used as an
     element of a container. The ODB compiler uses the same mechanisms
     to access data members in composite value types as in object types.
     Composite value types are discussed in more detail in
     <a href="#7.2">Section 7.2, "Composite Value Types"</a>.</p>

  <h2><a name="3.3">3.3 Object and View Pointers</a></h2>

  <p>As we have seen in the previous chapter, some database operations
     create dynamically allocated instances of persistent classes and
     return pointers to these instances. As we will see in later chapters,
     pointers are also used to establish relationships between objects
     (<a href="#6">Chapter 6, "Relationships"</a>) as well as to cache
     persistent objects in a session (<a href="#11">Chapter 11,
     "Session"</a>). While in most cases you won't need to deal with
     pointers to views, it is possible to a obtain a dynamically allocated
     instance of a view using the <code>result_iterator::load()</code>
     function (<a href="#4.4">Section 4.4, "Query Results"</a>).</p>

  <p>By default, all these mechanisms use raw pointers to return
     objects and views as well as to pass and cache objects. This
     is normally sufficient for applications
     that have simple object lifetime requirements and do not use sessions
     or object relationships. In particular, a dynamically allocated object
     or view that is returned as a raw pointer from a database operation
     can be assigned to a smart pointer of our choice, for example
     <code>std::auto_ptr</code> (C++98/03 only), <code>std::unique_ptr</code>
     from C++11, or <code>shared_ptr</code> from C++11 or Boost.</p>

  <p>However, to avoid any possibility of a mistake, such as forgetting
     to use a smart pointer for a returned object or view, as well as to
     simplify the use of more advanced ODB functionality, such as sessions
     and bidirectional object relationships, it is recommended that you use
     smart pointers with the sharing semantics as object pointers.
     The <code>shared_ptr</code> smart pointer from C++11 or Boost
     is a good default choice. However, if sharing is not required and
     sessions are not used, then <code>std::unique_ptr</code> or
     <code>std::auto_ptr</code> can be used just as well.</p>

  <p>ODB provides several mechanisms for changing the object or view pointer
     type. To specify the pointer type on the per object or per view basis
     we can use the <code>db&nbsp;pointer</code> pragma, for example:</p>

  <pre class="cxx">
#pragma db object pointer(std::shared_ptr)
class person
{
  ...
};
  </pre>

  <p>We can also specify the default pointer for a group of objects or
     views at the namespace level:</p>

  <pre class="cxx">
#pragma db namespace pointer(std::shared_ptr)
namespace accounting
{
  #pragma db object
  class employee
  {
    ...
  };

  #pragma db object
  class employer
  {
    ...
  };
}
  </pre>

  <p>Finally, we can use the <code>--default-pointer</code> option to specify
     the default pointer for the whole file. Refer to the
     <a href="http://www.codesynthesis.com/products/odb/doc/odb.xhtml">ODB
     Compiler Command Line Manual</a> for details on this option's argument.
     The typical usage is shown below:</p>

  <pre class="terminal">
--default-pointer std::shared_ptr
  </pre>

  <p>An alternative to this method with the same effect is to specify the
     default pointer for the  global namespace:</p>

  <pre class="terminal">
#pragma db namespace() pointer(std::shared_ptr)
  </pre>

  <p>Note that we can always override the default pointer specified
     at the namespace level or with the command line option using
     the <code>db&nbsp;pointer</code> object or view pragma. For
     example:</p>

  <pre class="cxx">
#pragma db object pointer(std::shared_ptr)
namespace accounting
{
  #pragma db object
  class employee
  {
    ...
  };

  #pragma db object pointer(std::unique_ptr)
  class employer
  {
    ...
  };
}
  </pre>

  <p>Refer to <a href="#14.1.2">Section 14.1.2, "<code>pointer</code>
     (object)"</a>, <a href="#14.2.4">Section 14.2.4, "<code>pointer</code>
     (view)"</a>, and <a href="#14.5.1">Section 14.5.1, "<code>pointer</code>
     (namespace)"</a> for more information on these mechanisms.</p>

  <p>Built-in support that is provided by the ODB runtime library allows us
     to use <code>std::shared_ptr</code> (C++11),
     <code>std::unique_ptr</code> (C++11), or <code>std::auto_ptr</code>
     (C++98/03 only) as pointer types. Plus, ODB profile libraries, that are
     available for commonly used frameworks and libraries (such as Boost and
     Qt), provide support for smart pointers found in these frameworks and
     libraries (<a href="#III">Part III, "Profiles"</a>). It is also easy to
     add support for our own smart pointers, as described in
     <a href="#6.5"> Section 6.5, "Using Custom Smart Pointers"</a>.</p>

  <h2><a name="3.4">3.4 Database</a></h2>

  <p>Before an application can make use of persistence services
     offered by ODB, it has to create a database class instance. A
     database instance is the representation of the place where
     the application stores its persistent objects. We create
     a database instance by instantiating one of the database
     system-specific classes. For example, <code>odb::mysql::database</code>
     would be such a class for the MySQL database system. We will
     also normally pass a database name as an argument to the
     class' constructor. The following code fragment
     shows how we can create a database instance for the MySQL
     database system:</p>

  <pre class="cxx">
#include &lt;odb/database.hxx>
#include &lt;odb/mysql/database.hxx>

unique_ptr&lt;odb::database> db (
  new odb::mysql::database (
    "test_user"     // database login name
    "test_password" // database password
    "test_database" // database name
    ));
  </pre>

  <p>The <code>odb::database</code> class is a common interface for
     all the database system-specific classes provided by ODB. You
     would normally work with the database
     instance via this interface unless there is a specific
     functionality that your application depends on and which is
     only exposed by a particular system's <code>database</code>
     class. You will need to include the <code>&lt;odb/database.hxx></code>
     header file to make this class available in your application.</p>

  <p>The <code>odb::database</code> interface defines functions for
     starting transactions and manipulating persistent objects.
     These are discussed in detail in the remainder of this chapter
     as well as the next chapter which is dedicated to the topic of
     querying the database for persistent objects. For details on the
     system-specific <code>database</code> classes, refer to
     <a href="#II">Part II, "Database Systems"</a>.</p>

  <p>Before we can persist our objects, the corresponding database schema has
     to be created in the database. The schema contains table definitions and
     other relational database artifacts that are used to store the state of
     persistent objects in the database.</p>

  <p>There are several ways to create the database schema. The easiest is to
     instruct the ODB compiler to generate the corresponding schema from the
     persistent classes (<code>--generate-schema</code> option). The ODB
     compiler can generate the schema as a standalone SQL file,
     embedded into the generated C++ code, or as a separate C++ source file
     (<code>--schema-format</code> option). If we are using the SQL file
     to create the database schema, then this file should be executed,
     normally only once, before the application is started.</p>

  <p>Alternatively, if the schema is embedded directly into the generated
     code or produced as a separate C++ source file, then we can use the
     <code>odb::schema_catalog</code> class to create it in the database
     from within our application, for example:</p>

  <pre class="cxx">
#include &lt;odb/schema-catalog.hxx>

odb::transaction t (db->begin ());
odb::schema_catalog::create_schema (*db);
t.commit ();
  </pre>

  <p>Refer to the next section for information on the
     <code>odb::transaction</code> class.  The complete version of the above
     code fragment is available in the <code>schema/embedded</code> example in
     the <code>odb-examples</code> package.</p>

  <p>The <code>odb::schema_catalog</code> class has the following interface.
     You will need to include the <code>&lt;odb/schema-catalog.hxx></code>
     header file to make this class available in your application.</p>

  <pre class="cxx">
namespace odb
{
  class schema_catalog
  {
  public:
    static void
    create_schema (database&amp;,
                   const std::string&amp; name = "",
                   bool drop = true);

    static void
    drop_schema (database&amp;, const std::string&amp; name = "");

    static bool
    exists (database_id, const std::string&amp; name = "");

    static bool
    exists (const database&amp;, const std::string&amp; name = "")
  };
}
  </pre>

  <p>The first argument to the <code>create_schema()</code> function
     is the database instance that we would like to create the schema in.
     The second argument is the schema name. By default, the ODB
     compiler generates all embedded schemas with the default schema
     name (empty string). However, if your application needs to
     have several separate schemas, you can use the
     <code>--schema-name</code> ODB compiler option to assign
     custom schema names and then use these names as a second argument
     to <code>create_schema()</code>. By default, <code>create_schema()</code>
     will also delete all the database objects (tables, indexes, etc.) if
     they exist prior to creating the new ones. You can change this
     behavior by passing <code>false</code> as the third argument. The
     <code>drop_schema()</code> function allows you to delete all the
     database objects without creating the new ones.</p>

  <p>If the schema is not found, the <code>create_schema()</code> and
     <code>drop_schema()</code> functions throw the
     <code>odb::unknown_schema</code> exception. You can use the
     <code>exists()</code> function to check whether a schema for the
     specified database and with the specified name exists in the
     catalog. Note also that the <code>create_schema()</code> and
     <code>drop_schema()</code> functions should be called within a
     transaction.</p>

  <p>ODB also provides support for database schema evolution. Similar
     to schema creation, schema migration statements can be generated
     either as standalone SQL files or embedded into the generated C++
     code. For more information on schema evolution support, refer to
     <a href="#13">Chapter 13, "Database Schema Evolution"</a>.</p>

  <p>Finally, we can also use a custom database schema with ODB. This approach
     can work similarly to the standalone SQL file described above except that
     the database schema is hand-written or produced by another program. Or we
     could execute custom SQL statements that create the schema directly from
     our application. To map persistent classes to custom database schemas, ODB
     provides a wide range of mapping customization pragmas, such
     as <code>db&nbsp;table</code>, <code>db&nbsp;column</code>,
     and <code>db&nbsp;type</code> (<a href="#14">Chapter 14, "ODB Pragma
     Language"</a>). For sample code that shows how to perform such mapping
     for various C++ constructs, refer to the <code>schema/custom</code>
     example in the <code>odb-examples</code> package.</p>

  <h2><a name="3.5">3.5 Transactions</a></h2>

  <p>A transaction is an atomic, consistent, isolated and durable
     (ACID) unit of work. Database operations can only be
     performed within a transaction and each thread of execution
     in an application can have only one active transaction at a
     time.</p>

  <p>By atomicity we mean that when it comes to making changes to
     the database state within a transaction,
     either all the changes are applied or none at all. Consider,
     for example, a transaction that transfers funds between two
     objects representing bank accounts. If the debit function
     on the first object succeeds but the credit function on
     the second fails, the transaction is rolled back and the
     database state of the first object remains unchanged.</p>

  <p>By consistency we mean that a transaction must take all the
     objects stored in the database from one consistent state
     to another. For example, if a bank account object must
     reference a person object as its owner and we forget to
     set this reference before making the object persistent,
     the transaction will be rolled back and the database
     will remain unchanged.</p>

  <p>By isolation we mean that the changes made to the database
     state during a transaction are only visible inside this
     transaction until and unless it is committed. Using the
     above example with the bank transfer, the results of the
     debit operation performed on the first object is not
     visible to other transactions until the credit operation
     is successfully completed and the transaction is committed.</p>

  <p>By durability we mean that once the transaction is committed,
     the changes that it made to the database state are permanent
     and will survive failures such as an application crash. From
     now on the only way to alter this state is to execute and commit
     another transaction.</p>

  <p>A transaction is started by calling either the
     <code>database::begin()</code> or <code>connection::begin()</code>
     function. The returned transaction handle is stored in
     an instance of the <code>odb::transaction</code> class.
     You will need to include the <code>&lt;odb/transaction.hxx></code>
     header file to make this class available in your application.
     For example:</p>

  <pre class="cxx">
#include &lt;odb/transaction.hxx>

transaction t (db.begin ())

// Perform database operations.

t.commit ();
  </pre>

  <p>The <code>odb::transaction</code> class has the following
     interface:</p>

  <pre class="cxx">
namespace odb
{
  class transaction
  {
  public:
    using database_type = odb::database;
    using connection_type = odb::connection;

    explicit
    transaction (transaction_impl*, bool make_current = true);

    transaction ();

    void
    reset (transaction_impl*, bool make_current = true);

    void
    commit ();

    void
    rollback ();

    database_type&amp;
    database ();

    connection_type&amp;
    connection ();

    bool
    finilized () const;

  public:
    static bool
    has_current ();

    static transaction&amp;
    current ();

    static void
    current (transaction&amp;);

    static bool
    reset_current ();

    // Callback API.
    //
  public:
    ...
  };
}
  </pre>

  <p>The <code>commit()</code> function commits a transaction and
     <code>rollback()</code> rolls it back. Unless the transaction
     has been <em>finalized</em>, that is, explicitly committed or rolled
     back, the destructor of the <code>transaction</code> class will
     automatically roll it back when the transaction instance goes
     out of scope. If we try to commit or roll back a finalized
     transaction, the <code>odb::transaction_already_finalized</code>
     exception is thrown.</p>

  <p>The <code>database()</code> accessor returns the database this
     transaction is working on. Similarly, the <code>connection()</code>
     accessor returns the database connection this transaction is on
     (<a href="#3.6">Section 3.6, "Connections"</a>).</p>

  <p>The static <code>current()</code> accessor returns the
     currently active transaction for this thread. If there is no active
     transaction, this function throws the <code>odb::not_in_transaction</code>
     exception. We can check whether there is a transaction in effect in
     this thread using the <code>has_current()</code> static function.</p>

  <p>The <code>make_current</code> argument in the <code>transaction</code>
     constructor as well as the static <code>current()</code> modifier and
     <code>reset_current()</code> function give us additional
     control over the nomination of the currently active transaction.
     If we pass <code>false</code> as the <code>make_current</code>
     argument, then the newly created transaction will not
     automatically be made the active transaction for this
     thread. Later, we can use the static <code>current()</code> modifier
     to set this transaction as the active transaction.
     The <code>reset_current()</code> static function clears the
     currently active transaction. Together, these mechanisms
     allow for more advanced use cases, such as multiplexing
     two or more transactions on the same thread. For example:</p>

  <pre class="cxx">
transaction t1 (db1.begin ());        // Active transaction.
transaction t2 (db2.begin (), false); // Not active.

// Perform database operations on db1.

transaction::current (t2);            // Deactivate t1, activate t2.

// Perform database operations on db2.

transaction::current (t1);            // Switch back to t1.

// Perform some more database operations on db1.

t1.commit ();

transaction::current (t2);            // Switch to t2.

// Perform some more database operations on db2.

t2.commit ();
  </pre>

  <p>The <code>reset()</code> modifier allows us to reuse the same
     <code>transaction</code> instance to complete several database
     transactions. Similar to the destructor, <code>reset()</code>
     will roll the current transaction back if it hasn't been finalized.
     The default <code>transaction</code> constructor creates a finalized
     transaction which can later be initialized using <code>reset()</code>.
     The <code>finilized()</code> accessor can be used to check whether the
     transaction has been finalized. Here is how we can use this functionality
     to commit the current transaction and start a new one every time a
     certain number of database operations has been performed:</p>

  <pre class="cxx">
transaction t (db.begin ());

for (size_t i (0); i &lt; n; ++i)
{
  // Perform a database operation, such as persist an object.

  // Commit the current transaction and start a new one after
  // every 100 operations.
  //
  if (i % 100 == 0)
  {
    t.commit ();
    t.reset (db.begin ());
  }
}

t.commit ();
  </pre>

  <p>For more information on the transaction callback support, refer
     to <a href="#15.1">Section 15.1, "Transaction Callbacks"</a>.</p>

  <p>Note that in the above discussion of atomicity, consistency,
     isolation, and durability, all of those guarantees only apply
     to the object's state in the database as opposed to the object's
     state in the application's memory. It is possible to roll
     a transaction back but still have changes from this
     transaction in the application's memory. An easy way to
     avoid this potential inconsistency is to instantiate
     persistent objects only within the transaction scope. Consider,
     for example, these two implementations of the same transaction:</p>

  <pre class="cxx">
void
update_age (database&amp; db, person&amp; p)
{
  transaction t (db.begin ());

  p.age (p.age () + 1);
  db.update (p);

  t.commit ();
}
  </pre>

  <p>In the above implementation, if the <code>update()</code> call fails
     and the transaction is rolled back, the state of the <code>person</code>
     object in the database and the state of the same object in the
     application's memory will differ. Now consider an
     alternative implementation which only instantiates the
     <code>person</code> object for the duration of the transaction:</p>

  <pre class="cxx">
void
update_age (database&amp; db, unsigned long long id)
{
  transaction t (db.begin ());

  unique_ptr&lt;person> p (db.load&lt;person> (id));
  p.age (p.age () + 1);
  db.update (p);

  t.commit ();
}
  </pre>

  <p>Of course, it may not always be possible to write the
     application in this style. Oftentimes we need to access and
     modify the application's state of persistent objects out of
     transactions. In this case it may make sense to try to
     roll back the changes made to the application state if
     the transaction was rolled back and the database state
     remains unchanged. One way to do this is to re-load
     the object's state from the database, for example:</p>

  <pre class="cxx">
void
update_age (database&amp; db, person&amp; p)
{
  try
  {
    transaction t (db.begin ());

    p.age (p.age () + 1);
    db.update (p);

    t.commit ();
  }
  catch (...)
  {
    transaction t (db.begin ());
    db.load (p.id (), p);
    t.commit ();

    throw;
  }
}
  </pre>

  <p>See also <a href="#15.1">Section 15.1, "Transaction Callbacks"</a>
     for an alternative approach.</p>

  <h2><a name="3.6">3.6 Connections</a></h2>

  <p>The <code>odb::connection</code> class represents a connection
     to the database. Normally, you wouldn't work with connections
     directly but rather let the ODB runtime obtain and release
     connections as needed. However, certain use cases may require
     obtaining a connection manually. For completeness, this section
     describes the <code>connection</code> class and discusses some
     of its use cases. You may want to skip this section if you are
     reading through the manual for the first time.</p>

  <p>Similar to <code>odb::database</code>, the <code>odb::connection</code>
     class is a common interface for all the database system-specific
     classes provided by ODB. For details on the system-specific
     <code>connection</code> classes, refer to <a href="#II">Part II,
     "Database Systems"</a>.</p>

  <p>To make the <code>odb::connection</code> class available in your
     application you will need to include the <code>&lt;odb/connection.hxx></code>
     header file. The <code>odb::connection</code> class has the
     following interface:</p>

  <pre class="cxx">
namespace odb
{
  class connection
  {
  public:
    using database_type = odb::database;

    transaction
    begin () = 0;

    unsigned long long
    execute (const char* statement);

    unsigned long long
    execute (const std::string&amp; statement);

    unsigned long long
    execute (const char* statement, std::size_t length);

    database_type&amp;
    database ();
  };

  using connection_ptr = details::shared_ptr&lt;connection>;
}
  </pre>

  <p>The <code>begin()</code> function is used to start a transaction
     on the connection. The <code>execute()</code> functions allow
     us to execute native database statements on the connection.
     Their semantics are equivalent to the <code>database::execute()</code>
     functions (<a href="#3.12">Section 3.12, "Executing Native SQL
     Statements"</a>) except that they can be legally called outside
     a transaction. Finally, the <code>database()</code> accessor
     returns a reference to the <code>odb::database</code> instance
     to which this connection corresponds.</p>

  <p>To obtain a connection we call the <code>database::connection()</code>
     function. The connection is returned as <code>odb::connection_ptr</code>,
     which is an implementation-specific smart pointer with the shared
     pointer semantics. This, in particular, means that the connection
     pointer can be copied and returned from functions. Once the last
     instance of <code>connection_ptr</code> pointing to the same
     connection is destroyed, the connection is returned to the
     <code>database</code> instance. The following code fragment
     shows how we can obtain, use, and release a connection:</p>

  <pre class="cxx">
using namespace odb::core;

database&amp; db = ...
connection_ptr c (db.connection ());

// Temporarily disable foreign key constraints.
//
c->execute ("SET FOREIGN_KEY_CHECKS = 0");

// Start a transaction on this connection.
//
transaction t (c->begin ());
...
t.commit ();

// Restore foreign key constraints.
//
c->execute ("SET FOREIGN_KEY_CHECKS = 1");

// When 'c' goes out of scope, the connection is returned to 'db'.
  </pre>

  <p>Some of the use cases which may require direct manipulation of
     connections include out-of-transaction statement execution,
     such as the execution of connection configuration statements,
     the implementation of a connection-per-thread policy, and making
     sure that a set of transactions is executed on the same
     connection.</p>

  <h2><a name="3.7">3.7 Error Handling and Recovery</a></h2>

  <p>ODB uses C++ exceptions to report database operation errors. Most
     ODB exceptions signify <em>hard</em> errors or errors that cannot
     be corrected without some intervention from the application.
     For example, if we try to load an object with an unknown object
     id, the <code>odb::object_not_persistent</code> exception is
     thrown. Our application may be able to correct this error, for
     instance, by obtaining a valid object id and trying again.
     The hard errors and corresponding ODB exceptions that can be
     thrown by each database function are described in the remainder
     of this chapter with <a href="#3.14">Section 3.14, "ODB Exceptions"</a>
     providing a quick reference for all the ODB exceptions.</p>

  <p>The second group of ODB exceptions signify <em>soft</em> or
     <em>recoverable</em> errors. Such errors are temporary
     failures which normally can be corrected by simply re-executing
     the transaction. ODB defines three such exceptions:
     <code>odb::connection_lost</code>, <code>odb::timeout</code>,
     and <code>odb::deadlock</code>. All recoverable ODB exceptions
     are derived from the common <code>odb::recoverable</code> base
     exception which can be used to handle all the recoverable
     conditions with a single <code>catch</code> block.</p>

  <p>The <code>odb::connection_lost</code> exception is thrown if
     a connection to the database is lost in the middle of
     a transaction. In this situation the transaction is aborted but
     it can be re-tried without any changes. Similarly, the
     <code>odb::timeout</code> exception is thrown if one of the
     database operations or the whole transaction has timed out.
     Again, in this case the transaction is aborted but can be
     re-tried as is.</p>

  <p>If two or more transactions access or modify more than one object
     and are executed concurrently by different applications or by
     different threads within the same application, then it is possible
     that these transactions will try to access objects in an incompatible
     order and deadlock. The canonical example of a deadlock are
     two transactions in which the first has modified <code>object1</code>
     and is waiting for the second transaction to commit its changes to
     <code>object2</code> so that it can also update <code>object2</code>.
     At the same time the second transaction has modified <code>object2</code>
     and is waiting for the first transaction to commit its changes to
     <code>object1</code> because it also needs to modify <code>object1</code>.
     As a result, none of the two transactions can be completed.</p>

  <p>The database system detects such situations and automatically
     aborts the waiting operation in one of the deadlocked transactions.
     In ODB this translates to the <code>odb::deadlock</code>
     recoverable exception being thrown from one of the database functions.</p>

  <p>The following code fragment shows how to handle the recoverable
     exceptions by restarting the affected transaction:</p>

  <pre class="cxx">
const unsigned short max_retries = 5;

for (unsigned short retry_count (0); ; retry_count++)
{
  try
  {
    transaction t (db.begin ());

    ...

    t.commit ();
    break;
  }
  catch (const odb::recoverable&amp; e)
  {
    if (retry_count > max_retries)
      throw retry_limit_exceeded (e.what ());
    else
      continue;
  }
}
  </pre>

  <h2><a name="3.8">3.8 Making Objects Persistent</a></h2>

  <p>A newly created instance of a persistent class is transient.
     We use the <code>database::persist()</code> function template
     to make a transient instance persistent. This function has four
     overloaded versions with the following signatures:</p>

  <pre class="cxx">
  template &lt;typename T>
  typename object_traits&lt;T>::id_type
  persist (const T&amp; object);

  template &lt;typename T>
  typename object_traits&lt;T>::id_type
  persist (const object_traits&lt;T>::const_pointer_type&amp; object);

  template &lt;typename T>
  typename object_traits&lt;T>::id_type
  persist (T&amp; object);

  template &lt;typename T>
  typename object_traits&lt;T>::id_type
  persist (const object_traits&lt;T>::pointer_type&amp; object);
  </pre>

  <p>Here and in the rest of the manual,
     <code>object_traits&lt;T>::pointer_type</code> and
     <code>object_traits&lt;T>::const_pointer_type</code> denote the
     unrestricted and constant object pointer types (<a href="#3.3">Section
     3.3, "Object and View Pointers"</a>), respectively.
     Similarly, <code>object_traits&lt;T>::id_type</code> denotes the object
     id type. The <code>odb::object_traits</code> template is part of the
     database support code generated by the ODB compiler.</p>

  <p>The first <code>persist()</code> function expects a constant reference
     to an instance being persisted. The second function expects a constant
     object pointer. Both of these functions can only be used on objects with
     application-assigned object ids (<a href="#14.4.2">Section 14.4.2,
     "<code>auto</code>"</a>).</p>

  <p>The second and third <code>persist()</code> functions are similar to the
     first two except that they operate on unrestricted references and object
     pointers. If the identifier of the object being persisted is assigned
     by the database, these functions update the id member of the passed
     instance with the assigned value. All four functions return the object
     id of the newly persisted object.</p>

  <p>If the database already contains an object of this type with this
     identifier, the <code>persist()</code> functions throw the
     <code>odb::object_already_persistent</code> exception. This should
     never happen for database-assigned object ids as long as the
     number of objects persisted does not exceed the value space of
     the id type.</p>

  <p>When calling the <code>persist()</code> functions, we don't need to
     explicitly specify the template type since it will be automatically
     deduced from the argument being passed. The following example shows
     how we can call these functions:</p>

  <pre class="cxx">
person john ("John", "Doe", 33);
shared_ptr&lt;person> jane (new person ("Jane", "Doe", 32));

transaction t (db.begin ());

db.persist (john);
unsigned long long jane_id (db.persist (jane));

t.commit ();

cerr &lt;&lt; "Jane's id: " &lt;&lt; jane_id &lt;&lt; endl;
  </pre>

  <p>Notice that in the above code fragment we have created instances
     that we were planning to make persistent before starting the
     transaction. Likewise, we printed Jane's id after we have committed
     the transaction. As a general rule, you should avoid performing
     operations within the transaction scope that can be performed
     before the transaction starts or after it terminates. An active
     transaction consumes both your application's resources, such as
     a database connection, as well as the database server's
     resources, such as object locks. By following the above rule you
     make sure these resources are released and made available to other
     threads in your application and to other applications as soon as
     possible.</p>

  <p>Some database systems support persisting multiple objects with a
     single underlying statement execution which can result in significantly
     improved performance. For such database systems ODB provides
     bulk <code>persist()</code> functions. For details, refer to
     <a href="#15.3">Section 15.3, "Bulk Database Operations"</a>.</p>

  <h2><a name="3.9">3.9 Loading Persistent Objects</a></h2>

  <p>Once an object is made persistent, and you know its object id, it
     can be loaded by the application using the <code>database::load()</code>
     function template. This function has two overloaded versions with
     the following signatures:</p>

  <pre class="cxx">
  template &lt;typename T>
  typename object_traits&lt;T>::pointer_type
  load (const typename object_traits&lt;T>::id_type&amp; id);

  template &lt;typename T>
  void
  load (const typename object_traits&lt;T>::id_type&amp; id, T&amp; object);
  </pre>

  <p>Given an object id, the first function allocates a new instance
     of the object class in the dynamic memory, loads its state from
     the database, and returns the pointer to the new instance. The
     second function loads the object's state into an existing instance.
     Both functions throw <code>odb::object_not_persistent</code> if
     there is no object of this type with this id in the database.</p>

  <p>When we call the first <code>load()</code> function, we need to
     explicitly specify the object type. We don't need to do this for
     the second function because the object type will be automatically
     deduced from the second argument, for example:</p>

  <pre class="cxx">
transaction t (db.begin ());

unique_ptr&lt;person> jane (db.load&lt;person> (jane_id));

db.load (jane_id, *jane);

t.commit ();
  </pre>

  <p>In certain situations it may be necessary to reload the state
     of an object from the database. While this is easy to achieve
     using the second <code>load()</code> function, ODB provides
     the <code>database::reload()</code> function template that
     has a number of special properties. This function has two
     overloaded versions with the following signatures:</p>

  <pre class="cxx">
  template &lt;typename T>
  void
  reload (T&amp; object);

  template &lt;typename T>
  void
  reload (const object_traits&lt;T>::pointer_type&amp; object);
  </pre>

  <p>The first <code>reload()</code> function expects an object
     reference, while the second expects an object pointer. Both
     functions expect the id member in the passed object to contain
     a valid object identifier and, similar to <code>load()</code>,
     both will throw <code>odb::object_not_persistent</code> if
     there is no object of this type with this id in the database.</p>

  <p>The first special property of <code>reload()</code>
     compared to the <code>load()</code> function is that it
     does not interact with the session's object cache
     (<a href="#11.1">Section 11.1, "Object Cache"</a>). That is, if
     the object being reloaded is already in the cache, then it will
     remain there after <code>reload()</code> returns. Similarly, if the
     object is not in the cache, then <code>reload()</code> won't
     put it there either.</p>

  <p>The second special property of the <code>reload()</code> function
     only manifests itself when operating on an object with the optimistic
     concurrency model. In this case, if the states of the object
     in the application memory and in the database are the same, then
     no reloading will occur. For more information on optimistic
     concurrency, refer to <a href="#12">Chapter 12, "Optimistic
     Concurrency"</a>.</p>

  <p>If we don't know for sure whether an object with a given id
     is persistent, we can use the <code>find()</code> function
     instead of <code>load()</code>, for example:</p>

  <pre class="cxx">
  template &lt;typename T>
  typename object_traits&lt;T>::pointer_type
  find (const typename object_traits&lt;T>::id_type&amp; id);

  template &lt;typename T>
  bool
  find (const typename object_traits&lt;T>::id_type&amp; id, T&amp; object);
  </pre>

  <p>If an object with this id is not found in the database, the first
     <code>find()</code> function returns a <code>NULL</code> pointer
     while the second function leaves the passed instance unmodified and
     returns <code>false</code>.</p>

  <p>If we don't know the object id, then we can use queries to
     find the object (or objects) matching some criteria
     (<a href="#4">Chapter 4, "Querying the Database"</a>). Note,
     however, that loading an object's state using its
     identifier can be significantly faster than executing a query.</p>


  <h2><a name="3.10">3.10 Updating Persistent Objects</a></h2>

  <p>If a persistent object has been modified, we can store the updated
     state in the database using the <code>database::update()</code>
     function template. This function has three overloaded versions with
     the following signatures:</p>

  <pre class="cxx">
  template &lt;typename T>
  void
  update (const T&amp; object);

  template &lt;typename T>
  void
  update (const object_traits&lt;T>::const_pointer_type&amp; object);

  template &lt;typename T>
  void
  update (const object_traits&lt;T>::pointer_type&amp; object);
  </pre>

  <p>The first <code>update()</code> function expects an object reference,
     while the other two expect object pointers. If the object passed to
     one of these functions does not exist in the database,
     <code>update()</code> throws the <code>odb::object_not_persistent</code>
     exception (but see a note on optimistic concurrency below).</p>

  <p>Below is an example of the funds transfer that we talked about
     in the earlier section on transactions. It uses the hypothetical
     <code>bank_account</code> persistent class:</p>

  <pre class="cxx">
void
transfer (database&amp; db,
          unsigned long long from_acc,
          unsigned long long to_acc,
          unsigned int amount)
{
  bank_account from, to;

  transaction t (db.begin ());

  db.load (from_acc, from);

  if (from.balance () &lt; amount)
    throw insufficient_funds ();

  db.load (to_acc, to);

  to.balance (to.balance () + amount);
  from.balance (from.balance () - amount);

  db.update (to);
  db.update (from);

  t.commit ();
}
  </pre>

  <p>The same can be accomplished using dynamically allocated objects
     and the <code>update()</code> function with object pointer argument,
     for example:</p>

  <pre class="cxx">
transaction t (db.begin ());

shared_ptr&lt;bank_account> from (db.load&lt;bank_account> (from_acc));

if (from->balance () &lt; amount)
  throw insufficient_funds ();

shared_ptr&lt;bank_account> to (db.load&lt;bank_account> (to_acc));

to->balance (to->balance () + amount);
from->balance (from->balance () - amount);

db.update (to);
db.update (from);

t.commit ();
  </pre>

  <p>If any of the <code>update()</code> functions are operating on a
     persistent class with the optimistic concurrency model, then they will
     throw the <code>odb::object_changed</code> exception if the state of the
     object in the database has changed since it was last loaded into the
     application memory. Furthermore, for such classes, <code>update()</code>
     no longer throws the <code>object_not_persistent</code> exception if
     there is no such object in the database. Instead, this condition is
     treated as a change of object state and <code>object_changed</code>
     is thrown instead. For a more detailed discussion of optimistic
     concurrency, refer to <a href="#12">Chapter 12, "Optimistic
     Concurrency"</a>.</p>

  <p>In ODB, persistent classes, composite value types, as well as individual
     data members can be declared read-only (see <a href="#14.1.4">Section
     14.1.4, "<code>readonly</code> (object)"</a>, <a href="#14.3.6">Section
     14.3.6, "<code>readonly</code> (composite value)"</a>, and
     <a href="#14.4.12">Section 14.4.12, "<code>readonly</code>
     (data member)"</a>).</p>

  <p>If an individual data member is declared read-only, then
     any changes to this member will be ignored when updating the database
     state of an object using any of the above <code>update()</code>
     functions. A <code>const</code> data member is automatically treated
     as read-only. If a composite value is declared read-only then all its
     data members are treated as read-only.</p>

  <p>If the whole object is declared read-only then the database state of
     this object cannot be changed. Calling any of the above
     <code>update()</code> functions for such an object will result in a
     compile-time error.</p>

  <p>Similar to <code>persist()</code>, for database systems that support
     this functionality, ODB provides bulk <code>update()</code> functions.
     For details, refer to <a href="#15.3">Section 15.3, "Bulk Database
     Operations"</a>.</p>

  <h2><a name="3.11">3.11 Deleting Persistent Objects</a></h2>

  <p>To delete a persistent object's state from the database we use the
     <code>database::erase()</code> or <code>database::erase_query()</code>
     function templates. If the application still has an instance of the
     erased object, this instance becomes transient. The <code>erase()</code>
     function has the following overloaded versions:</p>

  <pre class="cxx">
  template &lt;typename T>
  void
  erase (const T&amp; object);

  template &lt;typename T>
  void
  erase (const object_traits&lt;T>::const_pointer_type&amp; object);

  template &lt;typename T>
  void
  erase (const object_traits&lt;T>::pointer_type&amp; object);

  template &lt;typename T>
  void
  erase (const typename object_traits&lt;T>::id_type&amp; id);
  </pre>

  <p>The first <code>erase()</code> function uses an object itself, in
     the form of an object reference, to delete its state from the
     database. The next two functions accomplish the same result but using
     object pointers. Note that all three functions leave the passed
     object unchanged. It simply becomes transient. The last function
     uses the object id to identify the object to be deleted. If the
     object does not exist in the database, then all four functions
     throw the <code>odb::object_not_persistent</code> exception
     (but see a note on optimistic concurrency below).</p>

  <p>We have to specify the object type when calling the last
     <code>erase()</code> function. The same is unnecessary for the
     first three functions because the object type will be automatically
     deduced from their arguments. The following example shows how we
     can call these functions:</p>

  <pre class="cxx">
person&amp; john = ...
shared_ptr&lt;jane> jane = ...
unsigned long long joe_id = ...

transaction t (db.begin ());

db.erase (john);
db.erase (jane);
db.erase&lt;person> (joe_id);

t.commit ();
  </pre>

  <p>If any of the <code>erase()</code> functions except the last one are
     operating on a persistent class with the optimistic concurrency
     model, then they will throw the <code>odb::object_changed</code> exception
     if the state of the object in the database has changed since it was
     last loaded into the application memory. Furthermore, for such
     classes, <code>erase()</code> no longer throws the
     <code>object_not_persistent</code> exception if there is no such
     object in the database. Instead, this condition is treated as a
     change of object state and <code>object_changed</code> is thrown
     instead. For a more detailed discussion of optimistic concurrency,
     refer to <a href="#12">Chapter 12, "Optimistic Concurrency"</a>.</p>

  <p>Similar to <code>persist()</code> and <code>update()</code>, for
     database systems that support this functionality, ODB provides
     bulk <code>erase()</code> functions. For details, refer to
     <a href="#15.3">Section 15.3, "Bulk Database Operations"</a>.</p>

  <p>The <code>erase_query()</code> function allows us to delete
     the state of multiple objects matching certain criteria. It uses
     the query expression of the <code>database::query()</code> function
     (<a href="#4">Chapter 4, "Querying the Database"</a>) and,
     because the ODB query facility is optional, it is only available
     if the <code>--generate-query</code> ODB compiler option was
     specified. The <code>erase_query()</code> function has the
     following overloaded versions:</p>

  <pre class="cxx">
  template &lt;typename T>
  unsigned long long
  erase_query ();

  template &lt;typename T>
  unsigned long long
  erase_query (const odb::query&lt;T>&amp;);
  </pre>

  <p>The first <code>erase_query()</code> function is used to delete
     the state of all the persistent objects of a given type stored
     in the database. The second function uses the passed query instance
     to only delete the state of objects matching the query criteria.
     Both functions return the number of objects erased. When calling
     the <code>erase_query()</code> function, we have to explicitly
     specify the object type we are erasing. For example:</p>

  <pre class="cxx">
using query = odb::query&lt;person>;

transaction t (db.begin ());

db.erase_query&lt;person> (query::last == "Doe" &amp;&amp; query::age &lt; 30);

t.commit ();
  </pre>

  <p>Unlike the <code>query()</code> function, when calling
     <code>erase_query()</code> we cannot use members from pointed-to
     objects in the query expression. However, we can still use
     a member corresponding to a pointer as an ordinary object
     member that has the id type of the pointed-to object
     (<a href="#6">Chapter 6, "Relationships"</a>). This allows us
     to compare object ids as well as test the pointer for
     <code>NULL</code>. As an example, the following transaction
     makes sure that all the <code>employee</code> objects that
     reference an <code>employer</code> object that is about to
     be deleted are deleted as well. Here we assume that the
     <code>employee</code> class contains a pointer to the
     <code>employer</code> class. Refer to <a href="#6">Chapter 6,
     "Relationships"</a> for complete definitions of these
     classes.</p>

  <pre class="cxx">
using query = odb::query&lt;employee>;

transaction t (db.begin ());

employer&amp; e = ... // Employer object to be deleted.

db.erase_query&lt;employee> (query::employer == e.id ());
db.erase (e);

t.commit ();
  </pre>


  <h2><a name="3.12">3.12 Executing Native SQL Statements</a></h2>

  <p>In some situations we may need to execute native SQL statements
     instead of using the object-oriented database API described above.
     For example, we may want to tune the database schema generated
     by the ODB compiler or take advantage of a feature that is
     specific to the database system we are using. The
     <code>database::execute()</code> function, which has three
     overloaded versions, provides this functionality:</p>

  <pre class="cxx">
  unsigned long long
  execute (const char* statement);

  unsigned long long
  execute (const std::string&amp; statement);

  unsigned long long
  execute (const char* statement, std::size_t length)
  </pre>

  <p>The first <code>execute()</code> function expects the SQL statement
     as a zero-terminated C-string. The last version expects the explicit
     statement length as the second argument and the statement itself
     may contain <code>'\0'</code> characters, for example, to represent
     binary data, if the database system supports it. All three functions
     return the number of rows that were affected by the statement. For
     example:</p>

  <pre class="cxx">
transaction t (db.begin ());

db.execute ("DROP TABLE test");
db.execute ("CREATE TABLE test (n INT PRIMARY KEY)");

t.commit ();
  </pre>

  <p>While these functions must always be called within a transaction,
     it may be necessary to execute a native statement outside a
     transaction. This can be done using the
     <code>connection::execute()</code> functions as described in
     <a href="#3.6">Section 3.6, "Connections"</a>.</p>

  <h2><a name="3.13">3.13 Tracing SQL Statement Execution</a></h2>

  <p>Oftentimes it is useful to understand what SQL statements are
     executed as a result of high-level database operations. For
     example, we can use this information to figure out why certain
     transactions don't produce desired results or why they take
     longer than expected.</p>

  <p>While this information can usually be obtained from the database
     logs, ODB provides an application-side SQL statement tracing
     support that is both more convenient and finer-grained.
     For example, in a typical situation that calls for tracing
     we would like to see the SQL statements executed as a result
     of a specific transaction. While it may be difficult to
     extract such a subset of statements from the database logs,
     it is easy to achieve with ODB tracing support:</p>

  <pre class="cxx">
transaction t (db.begin ());
t.tracer (stderr_tracer);

...

t.commit ();
  </pre>

  <p>ODB allows us to specify a tracer on the database, connection,
     and transaction levels. If specified for the database, then
     all the statements executed on this database will be traced.
     On the other hand, if a tracer is specified for the
     connection, then only the SQL statements executed on this
     connection will be traced. Similarly, a tracer specified
     for a transaction will only show statements that are
     executed as part of this transaction. All three classes
     (<code>odb::database</code>, <code>odb::connection</code>,
      and <code>odb::transaction</code>) provide the identical
     tracing API:</p>

  <pre class="cxx">
  void
  tracer (odb::tracer&amp;);

  void
  tracer (odb::tracer*);

  odb::tracer*
  tracer () const;
  </pre>

  <p>The first two <code>tracer()</code> functions allow us to set
     the tracer object with the second one allowing us to clear the
     current tracer by passing a <code>NULL</code> pointer. The
     last <code>tracer()</code> function allows us to get the
     current tracer object. It returns a <code>NULL</code> pointer
     if there is no tracer in effect. Note that the tracing API
     does not manage the lifetime of the tracer object. The tracer
     should be valid for as long as it is being used. Furthermore,
     the tracing API is not thread-safe. Trying to set a tracer
     from multiple threads simultaneously will result in
     undefined behavior.</p>

  <p>The <code>odb::tracer</code> class defines a callback interface
     that can be used to create custom tracer implementations. The
     <code>odb::stderr_tracer</code> and <code>odb::stderr_full_tracer</code>
     are built-in tracer implementations provided by the ODB runtime.
     They both print SQL statements being executed to the standard error
     stream. The full tracer, in addition to tracing statement executions,
     also traces their preparations and deallocations. One situation where
     the full tracer can be particularly useful is if a statement (for
     example a custom query) contains a syntax error. In this case the
     error will be detected during preparation and, as a result, the
     statement will never be executed. The only way to see such a statement
     is by using the full tracing.</p>

  <p>The <code>odb::tracer</code> class is defined in the
     <code>&lt;odb/tracer.hxx></code> header file which you will need to
     include in order to make this class available in your application.
     The <code>odb::tracer</code> interface provided the following
     callback functions:</p>

  <pre class="cxx">
namespace odb
{
  class tracer
  {
  public:
    virtual void
    prepare (connection&amp;, const statement&amp;);

    virtual void
    execute (connection&amp;, const statement&amp;);

    virtual void
    execute (connection&amp;, const char* statement) = 0;

    virtual void
    deallocate (connection&amp;, const statement&amp;);
  };
}
  </pre>

  <p>The <code>prepare()</code> and <code>deallocate()</code> functions
     are called when a prepared statement is created and destroyed,
     respectively. The first <code>execute()</code> function is called
     when a prepared statement is executed while the second one is called
     when a normal statement is executed. The default implementations
     for the <code>prepare()</code> and <code>deallocate()</code>
     functions do nothing while the first <code>execute()</code> function
     calls the second one passing the statement text as the second
     argument. As a result, if all you are interested in are the
     SQL statements being executed, then you only need to override the
     second <code>execute()</code> function.</p>

  <p>In addition to the common <code>odb::tracer</code> interface,
     each database runtime provides a database-specific version
     as <code>odb::&lt;database>::tracer</code>. It has exactly
     the same interface as the common version except that the
     <code>connection</code> and <code>statement</code> types
     are database-specific, which gives us access to additional,
     database-specific information.</p>

  <p>As an example, consider a more elaborate, PostgreSQL-specific
     tracer implementation. Here we rely on the fact that the PostgreSQL
     ODB runtime uses names to identify prepared statements and this
     information can be obtained from the <code>odb::pgsql::statement</code>
     object:</p>

  <pre class="cxx">
#include &lt;odb/pgsql/tracer.hxx>
#include &lt;odb/pgsql/database.hxx>
#include &lt;odb/pgsql/connection.hxx>
#include &lt;odb/pgsql/statement.hxx>

class pgsql_tracer: public odb::pgsql::tracer
{
  virtual void
  prepare (odb::pgsql::connection&amp; c, const odb::pgsql::statement&amp; s)
  {
    cerr &lt;&lt; c.database ().db () &lt;&lt; ": PREPARE " &lt;&lt; s.name ()
         &lt;&lt; " AS " &lt;&lt; s.text () &lt;&lt; endl;
  }

  virtual void
  execute (odb::pgsql::connection&amp; c, const odb::pgsql::statement&amp; s)
  {
    cerr &lt;&lt; c.database ().db () &lt;&lt; ": EXECUTE " &lt;&lt; s.name () &lt;&lt; endl;
  }

  virtual void
  execute (odb::pgsql::connection&amp; c, const char* statement)
  {
    cerr &lt;&lt; c.database ().db () &lt;&lt; ": " &lt;&lt; statement &lt;&lt; endl;
  }

  virtual void
  deallocate (odb::pgsql::connection&amp; c, const odb::pgsql::statement&amp; s)
  {
    cerr &lt;&lt; c.database ().db () &lt;&lt; ": DEALLOCATE " &lt;&lt; s.name () &lt;&lt; endl;
  }
};
  </pre>

  <p>Note also that you can only set a database-specific tracer object
     using a database-specific database instance, for example:</p>

  <pre class="cxx">
pgsql_tracer tracer;

odb::database&amp; db = ...;
db.tracer (tracer); // Compile error.

odb::pgsql::database&amp; db = ...;
db.tracer (tracer); // Ok.
  </pre>

  <h2><a name="3.14">3.14 ODB Exceptions</a></h2>

  <p>In the previous sections we have already mentioned some of the
     exceptions that can be thrown by the database functions. In this
     section we will discuss the ODB exception hierarchy and document
     all the exceptions that can be thrown by the common ODB
     runtime.</p>

  <p>The root of the ODB exception hierarchy is the abstract
     <code>odb::exception</code> class. This class derives
     from <code>std::exception</code> and has the following
     interface:</p>

  <pre class="cxx">
namespace odb
{
  struct exception: std::exception
  {
    virtual const char*
    what () const throw () = 0;
  };
}
  </pre>

  <p>Catching this exception guarantees that we will catch all the
     exceptions thrown by ODB. The <code>what()</code> function
     returns a human-readable description of the condition that
     triggered the exception.</p>

  <p>The concrete exceptions that can be thrown by ODB are presented
     in the following listing:</p>

  <pre class="cxx">
namespace odb
{
  struct null_pointer: exception
  {
    virtual const char*
    what () const throw ();
  };

  // Transaction exceptions.
  //
  struct already_in_transaction: exception
  {
    virtual const char*
    what () const throw ();
  };

  struct not_in_transaction: exception
  {
    virtual const char*
    what () const throw ();
  };

  struct transaction_already_finalized: exception
  {
    virtual const char*
    what () const throw ();
  };

  // Session exceptions.
  //
  struct already_in_session: exception
  {
    virtual const char*
    what () const throw ();
  };

  struct not_in_session: exception
  {
    virtual const char*
    what () const throw ();
  };

  struct session_required: exception
  {
    virtual const char*
    what () const throw ();
  };

  // Database operations exceptions.
  //
  struct recoverable: exception
  {
  };

  struct connection_lost: recoverable
  {
    virtual const char*
    what () const throw ();
  };

  struct timeout: recoverable
  {
    virtual const char*
    what () const throw ();
  };

  struct deadlock: recoverable
  {
    virtual const char*
    what () const throw ();
  };

  struct object_not_persistent: exception
  {
    virtual const char*
    what () const throw ();
  };

  struct object_already_persistent: exception
  {
    virtual const char*
    what () const throw ();
  };

  struct object_changed: exception
  {
    virtual const char*
    what () const throw ();
  };

  struct result_not_cached: exception
  {
    virtual const char*
    what () const throw ();
  };

  struct database_exception: exception
  {
  };

  // Polymorphism support exceptions.
  //
  struct abstract_class: exception
  {
    virtual const char*
    what () const throw ();
  };

  struct no_type_info: exception
  {
    virtual const char*
    what () const throw ();
  };

  // Prepared query support exceptions.
  //
  struct prepared_already_cached: exception
  {
    const char*
    name () const;

    virtual const char*
    what () const throw ();
  };

  struct prepared_type_mismatch: exception
  {
    const char*
    name () const;

    virtual const char*
    what () const throw ();
  };

  // Schema catalog exceptions.
  //
  struct unknown_schema: exception
  {
    const std::string&amp;
    name () const;

    virtual const char*
    what () const throw ();
  };

  struct unknown_schema_version: exception
  {
    schema_version
    version () const;

    virtual const char*
    what () const throw ();
  };

  // Section exceptions.
  //
  struct section_not_loaded: exception
  {
    virtual const char*
    what () const throw ();
  };

  struct section_not_in_object: exception
  {
    virtual const char*
    what () const throw ();
  };

  // Bulk operation exceptions.
  //
  struct multiple_exceptions: exception
  {
    ...

    virtual const char*
    what () const throw ();
  };
}
  </pre>

  <p>The <code>null_pointer</code> exception is thrown when a
     pointer to a persistent object declared non-<code>NULL</code>
     with the <code>db&nbsp;not_null</code> or
     <code>db&nbsp;value_not_null</code> pragma has the <code>NULL</code>
     value. See <a href="#6">Chapter 6, "Relationships"</a> for details.</p>

  <p>The next three exceptions (<code>already_in_transaction</code>,
     <code>not_in_transaction</code>,
     <code>transaction_already_finalized</code>) are thrown by the
     <code>odb::transaction</code> class and are discussed
     in <a href="#3.5">Section 3.5, "Transactions"</a>.</p>

  <p>The next two exceptions (<code>already_in_session</code>, and
     <code>not_in_session</code>) are thrown by the <code>odb::session</code>
     class and are discussed in <a href="#11">Chapter 11, "Session"</a>.</p>

  <p>The <code>session_required</code> exception is thrown when ODB detects
     that correctly loading a bidirectional object relationship requires a
     session but one is not used. See <a href="#6.2">Section 6.2,
     "Bidirectional Relationships"</a> for more information on this
     exception.</p>

  <p>The <code>recoverable</code> exception serves as a common base
     for all the recoverable exceptions, which are: <code>connection_lost</code>,
     <code>timeout</code>, and <code>deadlock</code>. The
     <code>connection_lost</code> exception is thrown when a connection
     to the database is lost. Similarly, the <code>timeout</code> exception
     is thrown if one of the database operations or the whole transaction
     has timed out. The <code>deadlock</code> exception is thrown when a
     transaction deadlock is detected by the database system. These
     exceptions can be thrown by any database function. See
     <a href="#3.7">Section 3.7, "Error Handling and Recovery"</a>
     for details.</p>

  <p>The <code>object_already_persistent</code> exception is thrown
     by the <code>persist()</code> database function. See
     <a href="#3.8">Section 3.8, "Making Objects Persistent"</a>
     for details.</p>

  <p>The <code>object_not_persistent</code> exception is thrown
     by the <code>load()</code>, <code>update()</code>, and
     <code>erase()</code> database functions. Refer to
     <a href="#3.9">Section 3.9, "Loading Persistent Objects"</a>,
     <a href="#3.10">Section 3.10, "Updating Persistent Objects"</a>, and
     <a href="#3.11">Section 3.11, "Deleting Persistent Objects"</a> for
     more information.</p>

  <p>The <code>object_changed</code> exception is thrown
     by the <code>update()</code> database function and certain
     <code>erase()</code> database functions when
     operating on objects with the optimistic concurrency model. See
     <a href="#12">Chapter 12, "Optimistic Concurrency"</a> for details.</p>

  <p>The <code>result_not_cached</code> exception is thrown by
     the query result class. Refer to <a href="#4.4">Section 4.4,
     "Query Result"</a> for details.</p>

  <p>The <code>database_exception</code> exception is a base class for all
     database system-specific exceptions that are thrown by the
     database system-specific runtime library. Refer to <a href="#II">Part
     II, "Database Systems"</a> for more information.</p>

  <p>The <code>abstract_class</code> exception is thrown by the database
     functions when we attempt to persist, update, load, or erase an
     instance of a polymorphic abstract class. For more information
     on abstract classes, refer to <a href="#14.1.3">Section 14.1.3,
     "<code>abstract</code>"</a>.</p>

  <p>The <code>no_type_info</code> exception is thrown by the database
     functions when we attempt to persist, update, load, or erase an
     instance of a polymorphic class for which no type information
     is present in the application. This normally means that the
     generated database support code for this class has not been
     linked (or dynamically loaded) into the application or the
     discriminator value has not been mapped to a persistent
     class. For more information on polymorphism support, refer to
     <a href="#8.2">Section 8.2, "Polymorphism Inheritance"</a>.</p>

  <p>The <code>prepared_already_cached</code> exception is thrown by the
     <code>cache_query()</code> function if a prepared query with the
     specified name is already cached. The <code>prepared_type_mismatch</code>
     exception is thrown by the <code>lookup_query()</code> function if
     the specified prepared query object type or parameters type
     does not match the one in the cache. Refer to <a href="#4.5">Section
     4.5, "Prepared Queries"</a> for details.</p>

  <p>The <code>unknown_schema</code> exception is thrown by the
     <code>odb::schema_catalog</code> class if a schema with the specified
     name is not found. Refer to <a href="#3.4">Section 3.4, "Database"</a>
     for details. The <code>unknown_schema_version</code> exception is thrown
     by the <code>schema_catalog</code> functions that deal with database
     schema evolution if the passed or current version is unknow. Refer
     to <a href="#13">Chapter 13, "Database Schema Evolution"</a> for
     details.</p>

  <p>The <code>section_not_loaded</code> exception is thrown if we
     attempt to update an object section that hasn't been loaded.
     The <code>section_not_in_object</code> exception is thrown if
     the section instance being loaded or updated does not belong
     to the corresponding object. See <a href="#9">Chapter 9,
     "Sections"</a> for more information on these exceptions.</p>

  <p>The <code>multiple_exceptions</code> exception is thrown by the
     bulk API functions. Refer to <a href="#15.3">Section 15.3, "Bulk
     Database Operations"</a> for details.</p>

  <p>The <code>odb::exception</code> class is defined in the
     <code>&lt;odb/exception.hxx></code> header file. All the
     concrete ODB exceptions are defined in
     <code>&lt;odb/exceptions.hxx></code> which also includes
     <code>&lt;odb/exception.hxx></code>. Normally you don't
     need to include either of these two headers because they are
     automatically included by <code>&lt;odb/database.hxx></code>.
     However, if the source file that handles ODB exceptions
     does not include <code>&lt;odb/database.hxx></code>, then
     you will need to explicitly include one of these headers.</p>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="4">4 Querying the Database</a></h1>

  <p>If we don't know the identifiers of the objects that we are looking
     for, we can use queries to search the database for objects matching
     certain criteria. The ODB query facility is optional and we need to
     explicitly request the generation of the necessary database support
     code with the <code>--generate-query</code> ODB compiler option.</p>

  <p>ODB provides a flexible query API that offers two distinct levels of
     abstraction from the database system query language such as SQL.
     At the high level we are presented with an easy to use yet powerful
     object-oriented query language, called ODB Query Language. This
     query language is modeled after and is integrated into C++ allowing
     us to write expressive and safe queries that look and feel like
     ordinary C++. We have already seen examples of these queries in the
     introductory chapters. Below is another, more interesting, example:</p>

  <pre class="cxx">
  using query = odb::query&lt;person>;
  using result = odb::result&lt;person>;

  unsigned short age;
  query q (query::first == "John" &amp;&amp; query::age &lt; query::_ref (age));

  for (age = 10; age &lt; 100; age += 10)
  {
    result r (db.query&lt;person> (q));
    ...
  }
  </pre>

  <p>At the low level, queries can be written as predicates using
     the database system-native query language such as the
     <code>WHERE</code> predicate from the SQL <code>SELECT</code>
     statement. This language will be referred to as native query
     language. At this level ODB still takes care of converting
     query parameters from C++ to the database system format. Below
     is the re-implementation of the above example using SQL as
     the native query language:</p>

  <pre class="cxx">
  query q ("first = 'John' AND age = " + query::_ref (age));
  </pre>

  <p>Note that at this level we lose the static typing of
     query expressions. For example, if we wrote something like this:</p>

  <pre class="cxx">
  query q (query::first == 123 &amp;&amp; query::agee &lt; query::_ref (age));
  </pre>

  <p>We would get two errors during the C++ compilation. The first would
     indicate that we cannot compare <code>query::first</code> to an
     integer and the second would pick the misspelling in
     <code>query::agee</code>. On the other hand, if we wrote something
     like this:</p>

  <pre class="cxx">
  query q ("first = 123 AND agee = " + query::_ref (age));
  </pre>

  <p>It would compile fine and would trigger an error only when executed
     by the database system.</p>

  <p>We can also combine the two query languages in a single query, for
     example:</p>

  <pre class="cxx">
  query q ("first = 'John' AND" + (query::age &lt; query::_ref (age)));
  </pre>

  <h2><a name="4.1">4.1 ODB Query Language</a></h2>

  <p>An ODB query is an expression that tells the database system whether
     any given object matches the desired criteria. As such, a query expression
     always evaluates as <code>true</code> or <code>false</code>. At
     the higher level, an expression consists of other expressions
     combined with logical operators such as <code>&amp;&amp;</code> (AND),
     <code>||</code> (OR), and <code>!</code> (NOT). For example:</p>

  <pre class="cxx">
  using query = odb::query&lt;person>;

  query q (query::first == "John" || query::age == 31);
  </pre>

  <p>At the core of every query expression lie simple expressions which
     involve one or more object members, values, or parameters. To
     refer to an object member we use an expression such as
     <code>query::first</code> above. The names of members in the
     <code>query</code> class are derived from the names of data members
     in the object class by removing the common member name decorations,
     such as leading and trailing underscores, the <code>m_</code> prefix,
     etc.</p>

  <p>In a simple expression an object member can be compared to a value,
     parameter, or another member using a number of predefined operators
     and functions. The following table gives an overview of the available
     expressions:</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="operators" border="1">
    <tr>
      <th>Operator</th>
      <th>Description</th>
      <th>Example</th>
    </tr>

    <tr>
      <td><code>==</code></td>
      <td>equal</td>
      <td><code>query::age == 31</code></td>
    </tr>

    <tr>
      <td><code>!=</code></td>
      <td>unequal</td>
      <td><code>query::age != 31</code></td>
    </tr>

    <tr>
      <td><code>&lt;</code></td>
      <td>less than</td>
      <td><code>query::age &lt; 31</code></td>
    </tr>

    <tr>
      <td><code>></code></td>
      <td>greater than</td>
      <td><code>query::age > 31</code></td>
    </tr>

    <tr>
      <td><code>&lt;=</code></td>
      <td>less than or equal</td>
      <td><code>query::age &lt;= 31</code></td>
    </tr>

    <tr>
      <td><code>>=</code></td>
      <td>greater than or equal</td>
      <td><code>query::age >= 31</code></td>
    </tr>

    <tr>
      <td><code>in()</code></td>
      <td>one of the values</td>
      <td><code>query::age.in (30, 32, 34)</code></td>
    </tr>

    <tr>
      <td><code>in_range()</code></td>
      <td>one of the values in range</td>
      <td><code>query::age.in_range (begin, end)</code></td>
    </tr>

    <tr>
      <td><code>like()</code></td>
      <td>matches a pattern</td>
      <td><code>query::first.like ("J%")</code></td>
    </tr>

    <tr>
      <td><code>is_null()</code></td>
      <td>value is <code>NULL</code></td>
      <td><code>query::age.is_null ()</code></td>
    </tr>

    <tr>
      <td><code>is_not_null()</code></td>
      <td>value is <code>NOT NULL</code></td>
      <td><code>query::age.is_not_null ()</code></td>
    </tr>
  </table>

  <p>The <code>in()</code> function accepts a maximum of five arguments.
     Use the <code>in_range()</code> function if you need to compare
     to more than five values. This function accepts a pair of
     standard C++ iterators and compares to all the values from
     the <code>begin</code> position inclusive and until and
     excluding the <code>end</code> position. The following
     code fragment shows how we can use these functions:</p>

  <pre class="cxx">
  std::vector&lt;string> names;

  names.push_back ("John");
  names.push_back ("Jack");
  names.push_back ("Jane");

  query q1 (query::first.in ("John", "Jack", "Jane"));
  query q2 (query::first.in_range (names.begin (), names.end ()));
  </pre>

  <p>Note that the <code>like()</code> function does not perform any
     translation of the database system-specific extensions of the
     SQL <code>LIKE</code> operator. As a result, if you would like
     your application to be portable among various database systems,
     then limit the special characters used in the pattern to
     <code>%</code> (matches zero or more characters) and <code>_</code>
     (matches exactly one character). It is also possible to specify
     the escape character as a second argument to the <code>like()</code>
     function. This character can then be used to escape the special
     characters (<code>%</code> and <code>_</code>) in the pattern.
     For example, the following query will match any two characters
     separated by an underscore:</p>

  <pre class="cxx">
  query q (query::name.like ("_!__", "!"));
  </pre>

  <p>The operator precedence in the query expressions are the same
     as for equivalent C++ operators. We can use parentheses to
     make sure the expression is evaluated in the desired order.
     For example:</p>

  <pre class="cxx">
  query q ((query::first == "John" || query::first == "Jane") &amp;&amp;
           query::age &lt; 31);
  </pre>


  <h2><a name="4.2">4.2 Parameter Binding</a></h2>

  <p>An instance of the <code>odb::query</code> class encapsulates two
     parts of information about the query: the query expression and
     the query parameters. Parameters can be bound to C++ variables
     either by value or by reference.</p>

  <p>If a parameter is bound by value, then the value for this parameter
     is copied from the C++ variable to the query instance at the query
     construction time. On the other hand, if a parameter is bound by
     reference, then the query instance stores a reference to the
     bound variable. The actual value of the parameter is only extracted
     at the query execution time. Consider, for example, the following
     two queries:</p>

  <pre class="cxx">
  string name ("John");

  query q1 (query::first == query::_val (name));
  query q2 (query::first == query::_ref (name));

  name = "Jane";

  db.query&lt;person> (q1); // Find John.
  db.query&lt;person> (q2); // Find Jane.
  </pre>

  <p>The <code>odb::query</code> class provides two special functions,
     <code>_val()</code> and <code>_ref()</code>, that allow us to
     bind the parameter either by value or by reference, respectively.
     In the ODB query language, if the binding is not specified
     explicitly, the value semantic is used by default. In the
     native query language, binding must always be specified
     explicitly. For example:</p>

  <pre class="cxx">
  query q1 (query::age &lt; age);                // By value.
  query q2 (query::age &lt; query::_val (age));  // By value.
  query q3 (query::age &lt; query::_ref (age));  // By reference.

  query q4 ("age &lt; " + age);                  // Error.
  query q5 ("age &lt; " + query::_val (age));    // By value.
  query q6 ("age &lt; " + query::_ref (age));    // By reference.
  </pre>

  <p>A query that only has by-value parameters does not depend on any
     other variables and is self-sufficient once constructed. A query
     that has one or more by-reference parameters depends on the
     bound variables until the query is executed. If one such variable
     goes out of scope and we execute the query, the behavior is
     undefined.</p>

  <h2><a name="4.3">4.3 Executing a Query</a></h2>

  <p>Once we have the query instance ready and by-reference parameters
     initialized, we can execute the query using the
     <code>database::query()</code> function template. It has two
     overloaded versions:</p>

  <pre class="cxx">
  template &lt;typename T>
  result&lt;T>
  query (bool cache = true);

  template &lt;typename T>
  result&lt;T>
  query (const odb::query&lt;T>&amp;, bool cache = true);
  </pre>

  <p>The first <code>query()</code> function is used to return all the
     persistent objects of a given type stored in the database.
     The second function uses the passed query instance to only return
     objects matching the query criteria. The <code>cache</code> argument
     determines whether the objects' states should be cached in the
     application's memory or if they should be returned by the database
     system one by one as the iteration over the result progresses. The
     result caching is discussed in detail in the next section.</p>

  <p>When calling the <code>query()</code> function, we have to
     explicitly specify the object type we are querying. For example:</p>

  <pre class="cxx">
  using query = odb::query&lt;person>;
  using result = odb::result&lt;person>;

  result all (db.query&lt;person> ());
  result johns (db.query&lt;person> (query::first == "John"));
  </pre>

  <p>Note that it is not required to explicitly create a named
     query variable before executing it. For example, the following
     two queries are equivalent:</p>

  <pre class="cxx">
  query q (query::first == "John");

  result r1 (db.query&lt;person> (q));
  result r1 (db.query&lt;person> (query::first == "John"));
  </pre>

  <p>Normally, we would create a named query instance if we are
     planning to run the same query multiple times and would use the
     in-line version for those that are executed only once (see also
     <a href="#4.5">Section 4.5, "Prepared Queries"</a> for a more
     optimal way to re-execute the same query multiple times). A named
     query instance that does not have any by-reference parameters is
     immutable and can be shared between multiple threads without
     synchronization. On the other hand, a query instance with
     by-reference parameters is modified every time it is executed.
     If such a query is shared among multiple threads, then access
     to this query instance must be synchronized from the execution
     point and until the completion of the iteration over the result.</p>

  <p>It is also possible to create queries from other queries by
     combining them using logical operators. For example:</p>

  <pre class="cxx">
result
find_minors (database&amp; db, const query&amp; name_query)
{
  return db.query&lt;person> (name_query &amp;&amp; query::age &lt; 18);
}

result r (find_minors (db, query::first == "John"));
  </pre>

  <p>The result of executing a query is zero, one, or more objects
     matching the query criteria. The <code>query()</code> function
     returns this result as an instance of the <code>odb::result</code>
     class template, which provides a stream-like interface and is
     discussed in detail in the next section.</p>

  <p>In situations where we know that a query produces at most one
     element, we can instead use the <code>database::query_one()</code> and
     <code>database::query_value()</code> shortcut functions, for example:</p>

  <pre class="cxx">
  using query = odb::query&lt;person>;

  unique_ptr&lt;person> p (
    db.query_one&lt;person> (
      query::email == "jon@example.com"));
  </pre>

  <p>The shortcut query functions have the following signatures:</p>

  <pre class="cxx">
  template &lt;typename T>
  typename object_traits&lt;T>::pointer_type
  query_one ();

  template &lt;typename T>
  bool
  query_one (T&amp;);

  template &lt;typename T>
  T
  query_value ();

  template &lt;typename T>
  typename object_traits&lt;T>::pointer_type
  query_one (const odb::query&lt;T>&amp;);

  template &lt;typename T>
  bool
  query_one (const odb::query&lt;T>&amp;, T&amp;);

  template &lt;typename T>
  T
  query_value (const odb::query&lt;T>&amp;);
  </pre>

  <p>Similar to <code>query()</code>, the first three functions are used
     to return the only persistent object of a given type stored in the
     database. The second three versions use the passed query instance
     to only return the object matching the query criteria.</p>

  <p>Similar to the <code>database::find()</code> functions
     (<a href="#3.9">Section 3.9, "Loading Persistent Objects"</a>),
     <code>query_one()</code> can either allocate a new instance of the
     object class in the dynamic memory or it can load the object's state
     into an existing instance. The <code>query_value()</code> function
     allocates and returns the object by value.</p>

  <p>The <code>query_one()</code> function allows us to determine
     if the query result contains zero or one element. If no objects
     matching the query criteria were found in the database, the
     first version of <code>query_one()</code> returns the <code>NULL</code>
     pointer while the second &mdash; <code>false</code>. If the second
     version returns <code>false</code>, then the passed object
     remains unchanged. For example:</p>

  <pre class="cxx">
  if (unique_ptr&lt;person> p = db.query_one&lt;person> (
        query::email == "jon@example.com"))
  {
    ...
  }

  person p;
  if (db.query_one&lt;person> (query::email == "jon@example.com", p))
  {
    ...
  }
  </pre>

  <p>If the query executed using <code>query_one()</code> or
     <code>query_value()</code> returns more than one element,
     then these functions fail with an assertion. Additionally,
     <code>query_value()</code> also fails with an assertion if
     the query returned no elements.</p>

  <p>Common situations where we can use the shortcut functions are a
     query condition that uses a data member with the
     <code>unique</code> constraint (at most one element returned;
     see <a href="#14.7">Section 14.7, "Index Definition Pragmas"</a>)
     as well as aggregate queries (exactly one element returned; see
     <a href="#10">Chapter 10, "Views"</a>).</p>

  <h2><a name="4.4">4.4 Query Result</a></h2>

  <p>The <code>database::query()</code> function returns the result of
     executing a query as an instance of the <code>odb::result</code>
     class template, for example:</p>

  <pre class="cxx">
  using query = odb::query&lt;person>;
  using result = odb::result&lt;person>;

  result johns (db.query&lt;person> (query::first == "John"));
  </pre>

  <p>It is best to view an instance of <code>odb::result</code>
     as a handle to a stream, such as a socket stream. While we can
     make a copy of a result or assign one result to another, the
     two instances will refer to the same result stream. Advancing
     the current position in one instance will also advance it in
     another. The result instance is only usable within the transaction
     it was created in. Trying to manipulate the result after the
     transaction has terminated leads to undefined behavior.</p>

  <p>The <code>odb::result</code> class template conforms to the
     standard C++ sequence requirements and has the following
     interface:</p>

  <pre class="cxx">
namespace odb
{
  template &lt;typename T>
  class result
  {
  public:
    using iterator = odb::result_iterator&lt;T>;

  public:
    result ();

    result (const result&amp;);

    result&amp;
    operator= (const result&amp;);

    void
    swap (result&amp;)

  public:
    iterator
    begin ();

    iterator
    end ();

  public:
    void
    cache ();

    bool
    empty () const;

    std::size_t
    size () const;
  };
}
  </pre>

  <p>The default constructor creates an empty result set. The
    <code>cache()</code> function caches the returned objects'
    state in the application's memory. We have already mentioned
    result caching when we talked about query execution. As you
    may remember the <code>database::query()</code> function
    caches the result unless instructed not to by the caller.
    The <code>cache()</code> function allows us to
    cache the result at a later stage if it wasn't already
    cached during query execution.</p>

  <p>If the result is cached, the database state of all the returned
     objects is stored in the application's memory. Note that
     the actual objects are still only instantiated on demand
     during result iteration. It is the raw database state that
     is cached in memory. In contrast, for uncached results
     the object's state is sent by the database system one object
     at a time as the iteration progresses.</p>

  <p>Uncached results can improve the performance of both the application
     and the database system in situations where we have a large
     number of objects in the result or if we will only examine
     a small portion of the returned objects. However, uncached
     results have a number of limitations. There can only be one
     uncached result in a transaction. Creating another result
     (cached or uncached) by calling <code>database::query()</code>
     will invalidate the existing uncached result. Furthermore,
     calling any other database functions, such as <code>update()</code>
     or <code>erase()</code> will also invalidate the uncached result.
     It also follows that uncached results cannot be used on objects
     with containers (<a href="#5">Chapter 5, "Containers"</a>) since
     loading a container would invalidate the uncached result.</p>

  <p>The <code>empty()</code> function returns <code>true</code> if
     there are no objects in the result and <code>false</code> otherwise.
     The <code>size()</code> function can only be called for cached results.
     It returns the number of objects in the result. If we call this
     function on an uncached result, the <code>odb::result_not_cached</code>
     exception is thrown.</p>

  <p>To iterate over the objects in a result we use the
     <code>begin()</code> and <code>end()</code> functions
     together with the <code>odb::result&lt;T>::iterator</code>
     type, for example:</p>

  <pre class="cxx">
  result r (db.query&lt;person> (query::first == "John"));

  for (result::iterator i (r.begin ()); i != r.end (); ++i)
  {
    ...
  }
  </pre>

  <p>In C++11 we can use the <code>auto</code>-typed variabe instead
     of spelling the iterator type explicitly, for example:</p>

  <pre class="cxx">
  for (auto i (r.begin ()); i != r.end (); ++i)
  {
    ...
  }
  </pre>

  <p>The C++11 range-based <code>for</code>-loop can be used to further
     simplify the iteration:</p>

  <pre class="cxx">
  for (person&amp; p: r)
  {
    ...
  }
  </pre>

  <p>The result iterator is an input iterator which means that the
     only two position operations that it supports are to move to the
     next object and to determine whether the end of the result stream
     has been reached. In fact, the result iterator can only be in two
     states: the current position and the end position. If we have
     two iterators pointing to the current position and then we
     advance one of them, the other will advance as well. This,
     for example, means that it doesn't make sense to store an
     iterator that points to some object of interest in the result
     stream with the intent of dereferencing it after the iteration
     is over. Instead, we would need to store the object itself. We
     also cannot iterate over the same result multiple times without
     re-executing the query.</p>

  <p>The result iterator has the following dereference functions
     that can be used to access the pointed-to object:</p>

  <pre class="cxx">
namespace odb
{
  template &lt;typename T>
  class result_iterator
  {
  public:
    T*
    operator-> () const;

    T&amp;
    operator* () const;

    typename object_traits&lt;T>::pointer_type
    load ();

    void
    load (T&amp; x);

    typename object_traits&lt;T>::id_type
    id ();
  };
}
  </pre>

  <p>When we call the <code>*</code> or <code>-></code> operator,
     the iterator will allocate a new instance of the object class
     in the dynamic memory, load its state from the database
     state, and return a reference or pointer to the new instance. The
     iterator maintains the ownership of the returned object and will
     return the same pointer for subsequent calls to either of these
     operators until it is advanced to the next object or we call
     the first <code>load()</code> function (see below). For example:</p>

  <pre class="cxx">
  result r (db.query&lt;person> (query::first == "John"));

  for (result::iterator i (r.begin ()); i != r.end ();)
  {
    cout &lt;&lt; i->last () &lt;&lt; endl; // Create an object.
    person&amp; p (*i);             // Reference to the same object.
    cout &lt;&lt; p.age () &lt;&lt; endl;
    ++i;                        // Free the object.
  }
  </pre>

  <p>The overloaded <code>result_iterator::load()</code> functions are
     similar to <code>database::load()</code>. The first function
     returns a dynamically allocated instance of the current
     object. As an optimization, if the iterator already owns an object
     as a result of an earlier
     call to the <code>*</code> or <code>-></code> operator, then it
     relinquishes the ownership of this object and returns it instead.
     This allows us to write code like this without worrying about
     a double allocation:</p>

  <pre class="cxx">
  result r (db.query&lt;person> (query::first == "John"));

  for (result::iterator i (r.begin ()); i != r.end (); ++i)
  {
    if (i->last == "Doe")
    {
      unique_ptr p (i.load ());
      ...
    }
  }
  </pre>

  <p>Note, however, that because of this optimization, a subsequent
     to <code>load()</code> call to the <code>*</code> or <code>-></code>
     operator results in the allocation of a new object.</p>

  <p>The second <code>load()</code> function allows
     us to load the current object's state into an existing instance.
     For example:</p>

  <pre class="cxx">
  result r (db.query&lt;person> (query::first == "John"));

  person p;
  for (result::iterator i (r.begin ()); i != r.end (); ++i)
  {
    i.load (p);
    cout &lt;&lt; p.last () &lt;&lt; endl;
    cout &lt;&lt; i.age () &lt;&lt; endl;
  }
  </pre>

  <p>The <code>id()</code> function return the object id of the current
     object. While we can achieve the same by loading the object and getting
     its id, this function is more efficient since it doesn't actually
     create the object. This can be useful when all we need is the object's
     identifier. For example:</p>

  <pre class="cxx">
  std::set&lt;unsigned long long> set = ...; // Persons of interest.

  result r (db.query&lt;person> (query::first == "John"));

  for (result::iterator i (r.begin ()); i != r.end (); ++i)
  {
    if (set.find (i.id ()) != set.end ()) // No object loaded.
    {
      cout &lt;&lt; i->first () &lt;&lt; endl; // Object loaded.
    }
  }
  </pre>

  <h2><a name="4.5">4.5 Prepared Queries</a></h2>

  <p>Most modern relational database systems have the notion of a prepared
     statement. Prepared statements allow us to perform the potentially
     expensive tasks of parsing SQL, preparing the query execution
     plan, etc., once and then executing the same query multiple
     times, potentially using different values for parameters in
     each execution.</p>

  <p>In ODB all the non-query database operations such as
     <code>persist()</code>, <code>load()</code>, <code>update()</code>,
     etc., are implemented in terms of prepared statements that are cached
     and reused. While the <code>query()</code>, <code>query_one()</code>,
     and <code>query_value()</code> database operations also use prepared
     statements, these statements are not cached or reused by default since
     ODB has no knowledge of whether a query will be executed multiple times
     or only once. Instead, ODB provides a mechanism, called prepared queries,
     that allows us to prepare a query once and execute it multiple
     times. In other words, ODB prepared queries are a thin wrapper
     around the underlying database's prepared statement functionality.</p>

  <p>In most cases ODB shields the application developer from database
     connection management and multi-threading issues. However, when it
     comes to prepared queries, a basic understanding of how ODB manages
     these aspects is required. Conceptually, the <code>odb::database</code>
     class represents a specific database, that is, a data store. However,
     underneath, it maintains one or more connections to this database.
     A connection can be used only by a single thread at a time. When
     we start a transaction (by calling <code>database::begin()</code>),
     the transaction instance obtains a connection and holds on to it
     until the transaction is committed or rolled back. During this time
     no other thread can use this connection. When the transaction
     releases the connection, it may be closed or reused by another
     transaction in this or another thread. What exactly happens to
     a connection after it has been released depends on the connection
     factory that is used by the <code>odb::database</code> instance.
     For more information on connection factories, refer to
     <a href="#II">Part II, "Database Systems"</a>.</p>

  <p>A query prepared on one connection cannot be executed on another.
     In other words, a prepared query is associated with the connection.
     One important implication of this restriction is that we cannot
     prepare a query in one transaction and then try to execute it
     in another without making sure that both transactions use the
     same connection.</p>

  <p>To enable the prepared query functionality we need to specify
     the <code>--generate-prepared</code> ODB compiler option. If
     we are planning to always prepare our queries, then we can
     disable the once-off query execution support by also specifying
     the <code>--omit-unprepared</code> option.</p>

  <p>To prepare a query we use the <code>prepare_query()</code> function
     template. This function can be called on both the <code>odb::database</code>
     and <code>odb::connection</code> instances. The <code>odb::database</code>
     version simply obtains the connection used by the currently active
     transaction and calls the corresponding <code>odb::connection</code>
     version. If no transaction is currently active, then this function
     throws the <code>odb::not_in_transaction</code> exception
     (<a href="#3.5">Section 3.5, "Transactions"</a>). The
     <code>prepare_query()</code> function has the following signature:</p>

  <pre class="cxx">
  template &lt;typename T>
  prepared_query&lt;T>
  prepare_query (const char* name, const odb::query&lt;T>&amp;);
  </pre>

  <p>The first argument to the <code>prepare_query()</code> function is
     the prepared query name. This name is used as a key for prepared
     query caching (discussed later) and must be unique. For some databases,
     notably PostgreSQL, it is also used as a name of the underlying prepared
     statement. The name <code>"<i>object</i>_query"</code> (for example,
     <code>"person_query"</code>) is reserved for the once-off queries
     executed by the <code>database::query()</code> function. Note that
     the <code>prepare_query()</code> function makes only a shallow copy
     of this argument, which means that the name must be valid for the
     lifetime of the returned <code>prepared_query</code> instance.</p>

  <p>The second argument to the <code>prepare_query()</code> function
     is the query criteria. It has the same semantics as in the
     <code>query()</code> function discussed in <a href="#4.3">Section
     4.3, "Executing a Query"</a>. Similar to <code>query()</code>, we
     also have to explicitly specify the object type that we will be
     querying. For example:</p>

  <pre class="cxx">
using query = odb::query&lt;person>;
using prep_query = odb::prepared_query&lt;person>;

prep_query pq (
  db.prepare_query&lt;person> ("person-age-query", query::age > 50));
  </pre>

  <p>The result of executing the <code>prepare_query()</code> function is
     the <code>prepared_query</code> instance that represent the prepared
     query. It is best to view <code>prepared_query</code> as a handle to
     the underlying prepared statement. While we can make a copy of it or
     assign one <code>prepared_query</code> to another, the two instances
     will refer to the same prepared statement. Once the last instance of
     <code>prepared_query</code> referencing a specific prepared statement
     is destroyed, this statement is released. The <code>prepared_query</code>
     class template has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  template &lt;typename T>
  struct prepared_query
  {
    prepared_query ();

    prepared_query (const prepared_query&amp;)
    prepared_query&amp; operator= (const prepared_query&amp;)

    result&lt;T>
    execute (bool cache = true);

    typename object_traits&lt;T>::pointer_type
    execute_one ();

    bool
    execute_one (T&amp; object);

    T
    execute_value ();

    const char*
    name () const;

    statement&amp;
    statement () const;

    operator unspecified_bool_type () const;
  };
}
  </pre>

  <p>The default constructor creates an empty <code>prepared_query</code>
     instance, that is, an instance that does not reference a prepared
     statement and therefore cannot be executed. The only way to create
     a non-empty prepared query is by calling the <code>prepare_query()</code>
     function discussed above. To test whether the prepared query is empty,
     we can use the implicit conversion operator to a boolean type. For
     example:</p>

  <pre class="cxx">
  prepared_query&lt;person> pq;

  if (pq)
  {
    // Not empty.
    ...
  }
  </pre>

  <p>The <code>execute()</code> function executes the query and returns
     the result instance. The <code>cache</code> argument indicates
     whether the result should be cached and has the same semantics
     as in the <code>query()</code> function. In fact, conceptually,
     <code>prepare_query()</code> and <code>execute()</code> are just
     the <code>query()</code> function split into two:
     <code>prepare_query()</code> takes the first
     <code>query()</code> argument (the query condition) while
     <code>execute()</code> takes the second (the cache flag). Note
     also that re-executing a prepared query invalidates the
     previous execution result, whether cached or uncached. </p>

  <p>The <code>execute_one()</code> and <code>execute_value()</code>
     functions can be used as shortcuts to execute a query that is
     known to return at most one or exactly one object, respectively.
     The arguments and return values in these functions have the same
     semantics as in <code>query_one()</code> and <code>query_value()</code>.
     And similar to <code>execute()</code> above, <code>prepare_query()</code>
     and <code>execute_one/value()</code> can be seen as the
     <code>query_one/value()</code> function split into two:
     <code>prepare_query()</code> takes the first
     <code>query_one/value()</code> argument (the query condition) while
     <code>execute_one/value()</code> takes the second argument (if any)
     and returns the result. Note also that <code>execute_one/value()</code>
     never caches its result but invalidates the result of any previous
     <code>execute()</code> call on the same prepared query.</p>

  <p>The <code>name()</code> function returns the prepared query name.
     This is the same name as was passed as the first argument in the
     <code>prepare_query()</code> call. The <code>statement()</code>
     function returns a reference to the underlying prepared statement.
     Note also that calling any of these functions on an empty
     <code>prepared_query</code> instance results in undefined behavior.</p>

  <p>The simplest use-case for a prepared query is the need to
     execute the same query multiple times within a single transaction.
     Consider the following example that queries for people that are older
     than a number of different ages. This and subsequent code fragments
     are taken from the <code>prepared</code> example in the
     <code>odb-examples</code> package.</p>

  <pre class="cxx">
using query = odb::query&lt;person>;
using prep_query = odb::prepared_query&lt;person>;
using result = odb::result&lt;person>;

transaction t (db.begin ());

unsigned short age;
query q (query::age > query::_ref (age));
prep_query pq (db.prepare_query&lt;person> ("person-age-query", q));

for (age = 90; age > 40; age -= 10)
{
  result r (pq.execute ());
  ...
}

t.commit ();
  </pre>

  <p>Another scenario is the need to reuse the same query in multiple
     transactions that are executed at once. As was mentioned above,
     in this case we need to make sure that the prepared query and
     all the transactions use the same connection. Consider an
     alternative version of the above example that executes each
     query in a separate transaction:</p>

  <pre class="cxx">
connection_ptr conn (db.connection ());

unsigned short age;
query q (query::age > query::_ref (age));
prep_query pq (conn->prepare_query&lt;person> ("person-age-query", q));

for (age = 90; age > 40; age -= 10)
{
  transaction t (conn->begin ());

  result r (pq.execute ());
  ...

  t.commit ();
}
  </pre>


  <p>Note that with this approach we hold on to the database connection
     until all the transactions involving the prepared query are
     executed. In particular, this means that while we are busy, the
     connection cannot be reused by another thread. Therefore, this
     approach is only recommended if all the transactions are executed
     close to each other. Also note that an uncached (see below)
     prepared query is invalidated once we release the connection
     on which it was prepared.</p>

  <p>If we need to reuse a prepared query in transactions that are
     executed at various times, potentially in different threads, then
     the recommended approach is to cache the prepared query on the
     connection. To support this functionality the <code>odb::database</code>
     and <code>odb::connection</code> classes provide the following
     function templates. Similar to <code>prepare_query()</code>,
     the <code>odb::database</code> versions of the below
     functions call the corresponding <code>odb::connection</code>
     versions using the currently active transaction to resolve
     the connection.</p>

  <pre class="cxx">
  template &lt;typename T>
  void
  cache_query (const prepared_query&lt;T>&amp;);

  template &lt;typename T, typename P>
  void
  cache_query (const prepared_query&lt;T>&amp;,
               std::[auto|unique]_ptr&lt;P> params);

  template &lt;typename T>
  prepared_query&lt;T>
  lookup_query (const char* name);

  template &lt;typename T, typename P>
  prepared_query&lt;T>
  lookup_query (const char* name, P*&amp; params);
  </pre>

  <p>The <code>cache_query()</code> function caches the passed prepared
     query on the connection. The second overloaded version of
     <code>cache_query()</code> also takes a pointer to the
     by-reference query parameters. In C++98/03 it should be
     <code>std::auto_ptr</code> while in C++11
     &mdash; <code>std::unique_ptr</code>. The
     <code>cache_query()</code> function assumes ownership of the
     passed <code>params</code> argument. If a prepared query
     with the same name is already cached on this connection,
     then the <code>odb::prepared_already_cached</code> exception
     is thrown.</p>

  <p>The <code>lookup_query()</code> function looks up a previously
     cached prepared query given its name. The second overloaded
     version of <code>lookup_query()</code> also returns a pointer
     to the by-reference query parameters. If a prepared query
     with this name has not been cached, then an empty
     <code>prepared_query</code> instance is returned. If a
     prepared query with this name has been cached but either
     the object type or the parameters type does not match
     that which was cached, then the <code>odb::prepared_type_mismatch</code>
     exception is thrown.</p>

  <p>As a first example of the prepared query cache functionality,
     consider the case that does not use any by-reference parameters:</p>

  <pre class="cxx">
for (unsigned short i (0); i &lt; 5; ++i)
{
  transaction t (db.begin ());

  prep_query pq (db.lookup_query&lt;person> ("person-age-query"));

  if (!pq)
  {
    pq = db.prepare_query&lt;person> (
      "person-val-age-query", query::age > 50);
    db.cache_query (pq);
  }

  result r (pq.execute ());
  ...

  t.commit ();

  // Do some other work.
  //
  ...
}
  </pre>

  <p>The following example shows how to do the same but for a query that
     includes by-reference parameters. In this case the parameters are
     cached together with the prepared query.</p>

  <pre class="cxx">
for (unsigned short age (90); age > 40; age -= 10)
{
  transaction t (db.begin ());

  unsigned short* age_param;
  prep_query pq (
    db.lookup_query&lt;person> ("person-age-query", age_param));

  if (!pq)
  {
    unique_ptr&lt;unsigned short> p (new unsigned short);
    age_param = p.get ();
    query q (query::age > query::_ref (*age_param));
    pq = db.prepare_query&lt;person> ("person-age-query", q);
    db.cache_query (pq, p); // Assumes ownership of p.
  }

  *age_param = age; // Initialize the parameter.
  result r (pq.execute ());
  ...

  t.commit ();

  // Do some other work.
  //
  ...
}
  </pre>

  <p>As is evident from the above examples, when we use a prepared
     query cache, each transaction that executes a query must also
     include code that prepares and caches this query if it hasn't already
     been done. If a prepared query is used in a single place in the
     application, then this is normally not an issue since all the
     relevant code is kept in one place. However, if the same query
     is used in several different places in the application, then
     we may end up duplicating the same preparation and caching
     code, which makes it hard to maintain.</p>

  <p>To resolve this issue ODB allows us to register a prepared
     query factory that will be called to prepare and cache a
     query during the call to <code>lookup_query()</code>. To
     register a factory we use the <code>database::query_factory()</code>
     function. In C++98/03 it has the following signature:</p>

  <pre class="cxx">
  void
  query_factory (const char* name,
                 void (*factory) (const char* name, connection&amp;));
  </pre>

  <p>While in C++11 it uses the <code>std::function</code> class
     template:</p>

  <pre class="cxx">
  void
  query_factory (const char* name,
                 std::function&lt;void (const char* name, connection&amp;)>);
  </pre>

  <p>The first argument to the <code>query_factory()</code> function is
     the prepared query name that this factory will be called to prepare
     and cache. An empty name is treated as a fallback wildcard factory
     that is capable of preparing any query. The second argument is the
     factory function or, in C++11, function object or lambda.</p>

  <p>The example fragment shows how we can use the prepared query
     factory:</p>

  <pre class="cxx">
struct params
{
  unsigned short age;
  string first;
};

static void
query_factory (const char* name, connection&amp; c)
{
  unique_ptr&lt;params> p (new params);
  query q (query::age > query::_ref (p->age) &amp;&amp;
           query::first == query::_ref (p->first));
  prep_query pq (c.prepare_query&lt;person> (name, q));
  c.cache_query (pq, p);
}

db.query_factory ("person-age-name-query", &amp;query_factory);

for (unsigned short age (90); age > 40; age -= 10)
{
  transaction t (db.begin ());

  params* p;
  prep_query pq (db.lookup_query&lt;person> ("person-age-name-query", p));
  assert (pq);

  p->age = age;
  p->first = "John";
  result r (pq.execute ());
  ...

  t.commit ();
}
  </pre>

  <p>Instead of a static function we could have used a lambda:</p>

  <pre>
db.query_factory (
  "person-age-name-query",
  [] (const char* name, connection&amp; c)
  {
    unique_ptr&lt;params> p (new params);
    query q (query::age > query::_ref (p->age) &amp;&amp;
             query::first == query::_ref (p->first));
    prep_query pq (c.prepare_query&lt;person> (name, q));
    c.cache_query (pq, std::move (p));
  });
  </pre>

  Note that the <code>database::query_factory()</code> function is not
  thread-safe and should be called before starting any threads that may
  require this functionality. Normally, all the prepared query factories
  are registered as part of the database instance creation.

  <!-- CHAPTER -->

  <hr class="page-break"/>
  <h1><a name="5">5 Containers</a></h1>

  <p>The ODB runtime library provides built-in persistence support for all the
     commonly used standard C++98/03 containers, namely,
     <code>std::vector</code>, <code>std::list</code>, <code>std::deque</code>,
     <code>std::set</code>, <code>std::multiset</code>, <code>std::map</code>, and
     <code>std::multimap</code> as well as C++11 <code>std::array</code>,
     <code>std::forward_list</code>, <code>std::unordered_set</code>,
     <code>std::unordered_multiset</code>, <code>std::unordered_map</code>,
     and <code>std::unordered_multimap</code>.
     Plus, ODB profile libraries, that are
     available for commonly used frameworks and libraries (such as Boost and
     Qt), provide persistence support for containers found in these frameworks
     and libraries (<a href="#III">Part III, "Profiles"</a>). Both the
     ODB runtime library and profile libraries also provide a number of
     change-tracking container equivalents which can be used to minimize
     the number of database operations necessary to synchronize the container
     state with the database (<a href="#5.4">Section 5.4, "Change-Tracking
     Containers"</a>). It is also easy to persist custom container types
     as discussed later in <a href="#5.5">Section 5.5, "Using Custom
     Containers"</a>.</p>

  <p>We don't need to do anything special to declare a member of a
     container type in a persistent class. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
private:
  std::vector&lt;std::string> nicknames_;
  ...
};
  </pre>

  <p>The complete version of the above code fragment and the other code
     samples presented in this chapter can be found in the <code>container</code>
     example in the <code>odb-examples</code> package.</p>

  <p>A data member in a persistent class that is of a container type
     behaves like a value type. That is, when an object is made persistent,
     the elements of the container are stored in the database. Similarly,
     when a persistent object is loaded from the database, the contents
     of the container are automatically loaded as well. A data member
     of a container type can also use a smart pointer, as discussed
     in <a href="#7.3">Section 7.3, "Pointers and <code>NULL</code>
     Value Semantics"</a>.</p>

  <p>While an ordinary member is mapped to one or more columns in the
     object's table, a member of a container type is mapped to a separate
     table. The exact schema of such a table depends on the kind of
     container. ODB defines the following container kinds: ordered,
     set, multiset, map, and multimap. The container kinds and the
     contents of the tables to which they are mapped are discussed
     in detail in the following sections.</p>

  <p>Containers in ODB can contain simple value types (<a href="#7.1">Section
     7.1, "Simple Value Types"</a>), composite value types
     (<a href="#7.2">Section 7.2, "Composite Value Types"</a>), and pointers
     to objects (<a href="#6">Chapter 6, "Relationships"</a>). Containers of
     containers, either directly or indirectly via a composite value
     type, are not allowed. A key in a map or multimap container can
     be a simple or composite value type but not a pointer to an object.
     An index in the ordered container should be a simple integer value
     type.</p>

  <p>The value type in the ordered, set, and map containers as well as
     the key type in the map containers should be default-constructible.
     The default constructor in these types can be made private in which
     case the <code>odb::access</code> class should be made a friend of
     the value or key type. For example:</p>

  <pre class="cxx">
#pragma db value
class name
{
public:
  name (const std::string&amp;, const std::string&amp;);
  ...
private:
  friend class odb::access;
  name ();
  ...
};

#pragma db object
class person
{
  ...
private:
  std::vector&lt;name> aliases_;
  ...
};
  </pre>


  <h2><a name="5.1">5.1 Ordered Containers</a></h2>

  <p>In ODB an ordered container is any container that maintains (explicitly
     or implicitly) an order of its elements in the form of an integer index.
     Standard C++ containers that are ordered include <code>std::vector</code>
     <code>std::list</code>, and <code>std::deque</code> as well as C++11 <code>std::array</code> and
     <code>std::forward_list</code>. While elements in <code>std::set</code>
     are also kept in a specific order, this order is not based on an
     integer index but rather on the relationship between elements. As
     a result, <code>std::set</code> is not considered an ordered
     container for the purpose of persistence.</p>

  <p>The database table for an ordered container consists of at least
     three columns. The first column contains the object id of a
     persistent class instance of which the container is a member.
     The second column contains the element index within a container.
     And the last column contains the element value. If the object
     id or element value are composite, then, instead of a single
     column, they can occupy multiple columns. For an ordered
     container table the ODB compiler also defines two indexes:
     one for the object id column(s) and the other for the index
     column. Refer to <a href="#14.7">Section 14.7, "Index Definition
     Pragmas"</a> for more information on how to customize these
     indexes.</p>

  <p>Consider the following persistent object as an example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
private:
  #pragma db id auto
  unsigned long long id_;

  std::vector&lt;std::string> nicknames_;
  ...
};
  </pre>

  <p>The resulting database table (called <code>person_nicknames</code>) will
     contain the object id column of type <code>unsigned&nbsp;long</code>
     (called <code>object_id</code>), the index column of an integer type
     (called <code>index</code>), and the value column of type
     <code>std::string</code> (called <code>value</code>).</p>

  <p>A number of ODB pragmas allow us to customize the table name, column
     names, and native database types of an ordered container both, on
     the per-container and per-member basis. For more information on
     these pragmas, refer to <a href="#14">Chapter 14, "ODB Pragma
     Language"</a>. The following example shows some of the possible
     customizations:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
private:
  #pragma db table("nicknames")              \
             id_column("person_id")          \
             index_type("SMALLINT UNSIGNED") \
             index_column("nickname_number") \
             value_type("VARCHAR(255)")      \
             value_column("nickname")
  std::vector&lt;std::string> nicknames_;
  ...
};
  </pre>

  <p>While the C++ container used in a persistent class may be ordered,
     sometimes we may wish to store such a container in the database without
     the order information. In the example above, for instance, the order
     of person's nicknames is probably not important. To instruct the ODB
     compiler to ignore the order in ordered containers we can use the
     <code>db&nbsp;unordered</code> pragma (<a href="#14.3.9">Section 14.3.9,
     "<code>unordered</code>"</a>, <a href="#14.4.19">Section 14.4.19,
     "<code>unordered</code>"</a>). For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
private:
  #pragma db unordered
  std::vector&lt;std::string> nicknames_;
  ...
};
  </pre>

  <p>The table for an ordered container that is marked unordered won't
     have the index column and the order in which elements are retrieved
     from the database may not be the same as the order in which they
     were stored.</p>

  <h2><a name="5.2">5.2 Set and Multiset Containers</a></h2>

  <p>In ODB set and multiset containers (referred to as just set
     containers) are associative containers that contain elements
     based on some relationship between them. A set container may
     or may not guarantee a particular order of the elements that
     it stores. Standard C++ containers that are considered set
     containers for the purpose of persistence include
     <code>std::set</code> and <code>std::multiset</code> as well
     as C++11 <code>std::unordered_set</code> and
     <code>std::unordered_multiset</code>.</p>

  <p>The database table for a set container consists of at least
     two columns. The first column contains the object id of a
     persistent class instance of which the container is a member.
     And the second column contains the element value. If the object
     id or element value are composite, then, instead of a single
     column, they can occupy multiple columns. ODB compiler also
     defines an index on a set container table for the object id
     column(s). Refer to <a href="#14.7">Section 14.7, "Index Definition
     Pragmas"</a> for more information on how to customize this
     index.</p>

  <p>Consider the following persistent object as an example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
private:
  #pragma db id auto
  unsigned long long id_;

  std::set&lt;std::string> emails_;
  ...
};
  </pre>

  <p>The resulting database table (called <code>person_emails</code>) will
     contain the object id column of type <code>unsigned&nbsp;long</code>
     (called <code>object_id</code>) and the value column of type
     <code>std::string</code> (called <code>value</code>).</p>

  <p>A number of ODB pragmas allow us to customize the table name,
     column names, and native database types of a set container, both on
     the per-container and per-member basis. For more information on
     these pragmas, refer to <a href="#14">Chapter 14, "ODB Pragma
     Language"</a>. The following example shows some of the possible
     customizations:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
private:
  #pragma db table("emails")            \
             id_column("person_id")     \
             value_type("VARCHAR(255)") \
             value_column("email")
  std::set&lt;std::string> emails_;
  ...
};
  </pre>

  <h2><a name="5.3">5.3 Map and Multimap Containers</a></h2>

  <p>In ODB map and multimap containers (referred to as just map
     containers) are associative containers that contain key-value
     elements based on some relationship between keys. A map container
     may or may not guarantee a particular order of the elements that
     it stores. Standard C++ containers that are considered map
     containers for the purpose of persistence include
     <code>std::map</code> and <code>std::multimap</code> as well
     as C++11 <code>std::unordered_map</code> and
     <code>std::unordered_multimap</code>.</p>

  <p>The database table for a map container consists of at least
     three columns. The first column contains the object id of a
     persistent class instance of which the container is a member.
     The second column contains the element key. And the last column
     contains the element value. If the object id, element key, or
     element value are composite, then instead of a single column
     they can occupy multiple columns. ODB compiler also
     defines an index on a map container table for the object id
     column(s). Refer to <a href="#14.7">Section 14.7, "Index Definition
     Pragmas"</a> for more information on how to customize this
     index.</p>

  <p>Consider the following persistent object as an example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
private:
  #pragma db id auto
  unsigned long long id_;

  std::map&lt;unsigned short, float> age_weight_map_;
  ...
};
  </pre>

  <p>The resulting database table (called <code>person_age_weight_map</code>)
     will contain the object id column of type <code>unsigned&nbsp;long</code>
     (called <code>object_id</code>), the key column of type
     <code>unsigned short</code> (called <code>key</code>), and the value
     column of type <code>float</code> (called <code>value</code>).</p>

  <p>A number of ODB pragmas allow us to customize the table name,
     column names, and native database types of a map container, both on
     the per-container and per-member basis. For more information on
     these pragmas, refer to <a href="#14">Chapter 14, "ODB Pragma
     Language"</a>. The following example shows some of the possible
     customizations:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
private:
  #pragma db table("weight_map")      \
             id_column("person_id")   \
             key_type("INT UNSIGNED") \
             key_column("age")        \
             value_type("DOUBLE")     \
             value_column("weight")
  std::map&lt;unsigned short, float> age_weight_map_;
  ...
};
  </pre>

  <h2><a name="5.4">5.4 Change-Tracking Containers</a></h2>

  <p>When a persistent object containing one of the standard containers
     is updated in the database, ODB has no knowledge of which elements
     were inserted, erased, or modified. As a result, ODB has no choice
     but to assume the whole container has changed and update the state
     of every single element. This can result in a significant overhead
     if a container contains a large number of elements and we only
     changed a small subset of them.</p>

  <p>To eliminate this overhead, ODB provides a notion of <em>change-tracking
     containers</em>. A change-tracking container, besides containing
     its elements, just like an ordinary container, also includes the
     change state for each element. When it is time to update such a
     container in the database, ODB can use this change information to
     perform a minimum number of database operations necessary to
     synchronize the container state with the database.</p>

  <p>The current version of the ODB runtime library provides a change-tracking
     equivalent of <code>std::vector</code> (<a href="#5.4.1">Section 5.4.1,
     "Change-Tracking <code>vector</code>"</a>) with support for other
     standard container equivalents planned for future releases. ODB
     profile libraries also provide change-tracking equivalents for some
     containers found in the corresponding frameworks and libraries
     (<a href="#III">Part III, "Profiles"</a>).</p>

  <p>A change-tracking container equivalent can normally be used as a drop-in
     replacement for an ordinary container except for a few minor
     interface differences (discussed in the corresponding sub-sections).
     In particular, we don't need to do anything extra to effect
     change tracking. ODB will automatically start, stop, and reset
     change tracking when necessary. The following example illustrates
     this point using <code>odb::vector</code> as a replacement for
     <code>std::vector</code>.</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  odb::vector&lt;std::string> names;
};

person p; // No change tracking (not persistent).
p.names.push_back ("John Doe");

{
  transaction t (db.begin ());
  db.persist (p); // Start change tracking (persistent).
  t.commit ();
}

p.names.push_back ("Johnny Doo");

{
  transaction t (db.begin ());
  db.update (p); // One INSERT; reset change state.
  t.commit ();
}

p.names.modify (0) = "Doe, John"; // Instead of operator[].
p.names.pop_back ();

{
  transaction t (db.begin ());
  db.update (p); // One UPDATE, one DELETE; reset change state.
  t.commit ();
}

{
  transaction t (db.begin ());
  unique_ptr&lt;person> p1 (db.load&lt;person> (...)); // Start change tracking.
  p1->names.insert (p1->names.begin (), "Joe Do");
  db.update (*p1); // One UPDATE, one INSERT; reset change state.
  t.commit ();
}

{
  transaction t (db.begin ());
  db.erase (p); // One DELETE; stop change tracking (not persistent).
  t.commit ();
}
  </pre>

  <p>One interesting aspect of change tracking is what happens when a
     transaction that contains an update is later rolled back. In this
     case, while the change-tracking container has reset the change
     state (after update), actual changes were not committed to the
     database. Change-tracking containers handle this case by
     automatically registering a rollback callback and then, if it is
     called, marking the container as "completely changed". In this
     state, the container no longer tracks individual element changes
     and, when updated, falls back to the complete state update, just
     like an ordinary container. The following example illustrates
     this point:</p>

  <pre class="cxx">
person p;
p.names.push_back ("John Doe");

{
  transaction t (db.begin ());
  db.persist (p); // Start change tracking (persistent).
  t.commit ();
}

p.names.push_back ("Johnny Doo");

for (;;)
{
  try
  {
    transaction t (db.begin ());

    // First try: one INSERT.
    // Next try: one DELETE, two INSERTs.
    //
    db.update (p); // Reset change state.

    t.commit (); // If throws (rollback), mark as completely changed.
    break;
  }
  catch (const odb::recoverable&amp;)
  {
    continue;
  }
}
  </pre>

  <p>For the interaction of change-tracking containers with change-updated
     object sections, refer to <a href="#9.4">Section 9.4, "Sections and
     Change-Tracking Containers"</a>. Note also that change-tracking
     containers cannot be accessed with by-value accessors
     (<a href="#14.4.5">Section 14.4.5,
     "<code>get</code>/<code>set</code>/<code>access</code>"</a>)
     since in certain situations such access may involve a
     modification of the container (for example, clearing the change
     flag after update).</p>

  <h3><a name="5.4.1">5.4.1 Change-Tracking <code>vector</code></a></h3>

  <p>Class template <code>odb::vector</code>, defined in
     <code>&lt;odb/vector.hxx></code>, is a change-tracking
     equivalent for <code>std::vector</code>. It
     is implemented in terms of <code>std::vector</code> and is
     implicit-convertible to and implicit-constructible from
     <code>const std::vector&amp;</code>. In particular, this
     means that we can use <code>odb::vector</code> instance
     anywhere <code>const std::vector&amp;</code> is
     expected. In addition, <code>odb::vector</code> constant
     iterator (<code>const_iterator</code>) is the same type as
     that of <code>std::vector</code>.</p>

  <p><code>odb::vector</code> incurs 2-bit per element overhead
     in order to store the change state. It cannot
     be stored unordered in the database (<a href="#14.4.19">Section
     14.4.19 "<code>unordered</code>"</a>) but can be used as an inverse
     side of a relationship (<a href="#6.2">6.2 "Bidirectional
     Relationships"</a>). In this case, no change tracking is performed
     since no state for such a container is stored in the database.</p>

  <p>The number of database operations required to update the state
     of <code>odb::vector</code> corresponds well to the complexity
     of <code>std::vector</code> functions. In particular, adding or
     removing an element from the back of the vector (for example,
     with <code>push_back()</code> and <code>pop_back()</code>),
     requires only a single database statement execution. In contrast,
     inserting or erasing an element somewhere in the middle of the
     vector will require a database statement for every element that
     follows it.</p>

  <p><code>odb::vector</code> replicates most of the <code>std::vector</code>
     interface as defined in both C++98/03 and C++11 standards. However,
     functions and operators that provide direct write access to
     the elements had to be altered or disabled in order to support
     change tracking. Additional functions used to interface with
     <code>std::vector</code> and to control the change tracking state
     were also added. The following listing summarizes the differences
     between the <code>odb::vector</code> and <code>std::vector</code>
     interfaces. Any <code>std::vector</code> function or operator
     not mentioned in this listing has exactly the same signature
     and semantics in <code>odb::vector</code>. Functions and
     operators that were disabled are shown as commented out and
     are followed by functions/operators that replace them.</p>

  <pre class="cxx">
namespace odb
{
  template &lt;class T, class A = std::allocator&lt;T>>
  class vector
  {
    ...

    // Element access.
    //

    //reference operator[] (size_type);
      reference modify (size_type);

    //reference at (size_type);
      reference modify_at (size_type);

    //reference front ();
      reference modify_front ();

    //reference back ();
      reference modify_back ();

    //T*        data () noexcept;
      T*        modify_data () noexcept; // C++11 only.

    // Iterators.
    //
    using const_iterator = typename std::vector&lt;T, A>::const_iterator;

    class iterator
    {
      ...

      // Element Access.
      //

      //reference       operator* () const;
        const_reference operator* () const;
        reference       modify () const;

      //pointer       operator-> () const;
        const_pointer operator-> () const;

      //reference       operator[] (difference_type);
        const_reference operator[] (difference_type);
        reference       modify (difference_type) const;

      // Interfacing with std::vector::iterator.
      //
      typename std::vector&lt;T, A>::iterator base () const;
    };

    // Return std::vector iterators. The begin() functions mark
    // all the elements as modified.
    //
    typename std::vector&lt;T, A>::iterator         mbegin ();
    typename std::vector&lt;T, A>::iterator         mend ();
    typename std::vector&lt;T, A>::reverse_iterator mrbegin ();
    typename std::vector&lt;T, A>::reverse_iterator mrend ();

    // Interfacing with std::vector.
    //
    vector (const std::vector&lt;T, A>&amp;);
    vector (std::vector&lt;T, A>&amp;&amp;); // C++11 only.

    vector&amp; operator= (const std::vector&lt;T, A>&amp;);
    vector&amp; operator= (std::vector&lt;T, A>&amp;&amp;); // C++11 only.

    operator const std::vector&lt;T, A>&amp; () const;
    std::vector&lt;T, A>&amp; base ();
    const std::vector&lt;T, A>&amp; base ();

    // Change tracking.
    //
    bool _tracking () const;
    void _start () const;
    void _stop () const;
    void _arm (transaction&amp;) const;
  };
}
  </pre>

  <p>The following example highlights some of the differences between
     the two interfaces. <code>std::vector</code> versions are commented
     out.</p>

  <pre class="cxx">
#include &lt;vector>
#include &lt;odb/vector.hxx>

void f (const std::vector&lt;int>&amp;);

odb::vector&lt;int> v ({1, 2, 3});

f (v); // Ok, implicit conversion.

if (v[1] == 2) // Ok, const access.
  //v[1]++;
  v.modify (1)++;

//v.back () = 4;
v.modify_back () = 4;

for (auto i (v.begin ()); i != v.end (); ++i)
{
  if (*i != 0) // Ok, const access.
    //*i += 10;
    i.modify () += 10;
}

std::sort (v.mbegin (), v.mend ());
  </pre>

  <p>Note also the subtle difference between copy/move construction
     and copy/move assignment of <code>odb::vector</code> instances.
     While copy/move constructor will copy/move both the elements as
     well as their change state, in contrast, assignment is tracked
     as any other change to the vector content.</p>

  <h2><a name="5.5">5.5 Using Custom Containers</a></h2>

  <p>While the ODB runtime and profile libraries provide support for
     a wide range of containers, it is also easy to persist custom
     container types or make a change-tracking version out of one.</p>

  <p>To achieve this you will need to implement the
     <code>container_traits</code> class template specialization for
     your container. First, determine the container kind (ordered, set,
     multiset, map, or multimap) for your container type. Then use a
     specialization for one of the standard C++ containers found in
     the common ODB runtime library (<code>libodb</code>) as a base
     for your own implementation.</p>

  <p>Once the container traits specialization is ready for your container,
     you will need to include it into the ODB compilation process using
     the <code>--odb-epilogue</code> option and into the generated header
     files with the <code>--hxx-prologue</code> option. As an example,
     suppose we have a hash table container for which we have the traits
     specialization implemented in the <code>hashtable-traits.hxx</code>
     file. Then, we can create an ODB compiler options file for this
     container and save it to <code>hashtable.options</code>:</p>

  <pre>
# Options file for the hash table container.
#
--odb-epilogue '#include "hashtable-traits.hxx"'
--hxx-prologue '#include "hashtable-traits.hxx"'
  </pre>

  <p>Now, whenever we compile a header file that uses the hashtable
     container, we can specify the following command line option to
     make sure it is recognized by the ODB compiler as a container
     and the traits file is included in the generated code:</p>

  <pre>
--options-file hashtable.options
  </pre>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="6">6 Relationships</a></h1>

  <p>Relationships between persistent objects are expressed with pointers or
     containers of pointers. The ODB runtime library provides built-in support
     for <code>std::shared_ptr</code>/<code>std::weak_ptr</code> (C++11),
     <code>std::unique_ptr</code> (C++11),
     <code>std::auto_ptr</code> (C++98/03 only), and raw pointers. Plus, ODB
     profile libraries, that are available for commonly used frameworks and
     libraries (such as Boost and Qt), provide support for smart pointers
     found in these frameworks and libraries (<a href="#III">Part III,
     "Profiles"</a>). It is also easy to add support for a custom smart
     pointer as discussed later in <a href="#6.5"> Section 6.5, "Using Custom
     Smart Pointers"</a>. Any supported smart pointer can be used in a data
     member as long as it can be explicitly constructed from the canonical
     object pointer (<a href="#3.3">Section 3.3, "Object and View
     Pointers"</a>).  For example, we can use <code>weak_ptr</code> if the
     object pointer is <code>shared_ptr</code>.</p>

  <p>When an object containing a pointer to another object is loaded,
     the pointed-to object is loaded as well. In some situations this
     eager loading of the relationships is undesirable since it
     can lead to a large number of otherwise unused objects being
     instantiated from the database. To support finer control
     over relationships loading, the ODB runtime and profile
     libraries provide the so-called <em>lazy</em> versions of
     the supported pointers. An object pointed-to by a lazy pointer
     is not loaded automatically when the containing object is loaded.
     Instead, we have to explicitly request the instantiation of the
     pointed-to object. Lazy pointers are discussed in
     detail in <a href="#6.4">Section 6.4, "Lazy Pointers"</a>.</p>

  <p>As a simple example, consider the following employee-employer
     relationship. Code examples presented in this chapter
     will use the C++11 <code>shared_ptr</code> and <code>weak_ptr</code>
     smart pointers from the <code>std</code> namespace.</p>

  <pre class="cxx">
#pragma db object
class employer
{
  ...

  #pragma db id
  std::string name_;
};

#pragma db object
class employee
{
  ...

  #pragma db id
  unsigned long long id_;

  std::string first_name_;
  std::string last_name_;

  shared_ptr&lt;employer> employer_;
};
  </pre>

  <p>By default, an object pointer can be <code>NULL</code>. To
     specify that a pointer always points to a valid object we can
     use the <code>not_null</code> pragma (<a href="#14.4.6">Section
     14.4.6, "<code>null</code>/<code>not_null</code>"</a>) for
     single object pointers and the <code>value_not_null</code> pragma
     (<a href="#14.4.28">Section
     14.4.28, "<code>value_null</code>/<code>value_not_null</code>"</a>)
     for containers of object pointers. For example:</p>

  <pre class="cxx">
#pragma db object
class employee
{
  ...

  #pragma db not_null
  shared_ptr&lt;employer> current_employer_;

  #pragma db value_not_null
  std::vector&lt;shared_ptr&lt;employer>> previous_employers_;
};
  </pre>

  <p>In this case, if we call either <code>persist()</code> or
     <code>update()</code> database function on the
     <code>employee</code> object and the <code>current_employer_</code>
     pointer or one of the pointers stored in the
     <code>previous_employers_</code> container is <code>NULL</code>,
     then the <code>odb::null_pointer</code> exception will be thrown.</p>

  <p>We don't need to do anything special to establish or navigate a
     relationship between two persistent objects, as shown in the
     following code fragment:</p>

  <pre class="cxx">
// Create an employer and a few employees.
//
unsigned long long john_id, jane_id;
{
  shared_ptr&lt;employer> er (new employer ("Example Inc"));
  shared_ptr&lt;employee> john (new employee ("John", "Doe"));
  shared_ptr&lt;employee> jane (new employee ("Jane", "Doe"));

  john->employer_ = er;
  jane->employer_ = er;

  transaction t (db.begin ());

  db.persist (er);
  john_id = db.persist (john);
  jane_id = db.persist (jane);

  t.commit ();
}

// Load a few employee objects and print their employer.
//
{
  session s;
  transaction t (db.begin ());

  shared_ptr&lt;employee> john (db.load&lt;employee> (john_id));
  shared_ptr&lt;employee> jane (db.load&lt;employee> (jane_id));

  cout &lt;&lt; john->employer_->name_ &lt;&lt; endl;
  cout &lt;&lt; jane->employer_->name_ &lt;&lt; endl;

  t.commit ();
}
  </pre>

  <p>The only notable line in the above code is the creation of a
     session before the second transaction starts. As discussed in
     <a href="#11">Chapter 11, "Session"</a>, a session acts as a cache
     of persistent objects.
     By creating a session before loading the <code>employee</code>
     objects we make sure that their <code>employer_</code> pointers
     point to the same <code>employer</code> object. Without a
     session, each <code>employee</code> would have ended up pointing
     to its own, private instance of the Example Inc employer.</p>

  <p>As a general guideline, you should use a session when loading
     objects that have pointers to other persistent objects. A
     session makes sure that for a given object id, a single instance
     is shared among all other objects that relate to it.</p>

  <p>We can also use data members from pointed-to
     objects in database queries (<a href="#4">Chapter 4, "Querying the
     Database"</a>). For each pointer in a persistent class, the query
     class defines a smart pointer-like member that contains members
     corresponding to the data members in the pointed-to object. We
     can then use the access via a pointer syntax (<code>-></code>)
     to refer to data members in pointed-to objects.
     For example, the query class for the <code>employee</code> object
     contains the <code>employer</code> member (its name is derived from the
     <code>employer_</code> pointer) which in turn contains the
     <code>name</code> member (its name is derived from the
     <code>employer::name_</code> data member of the pointed-to object).
     As a result, we can use the <code>query::employer->name</code>
     expression while querying the database for the <code>employee</code>
     objects. For example, the following transaction finds all the
     employees of Example Inc that have the Doe last name:</p>

  <pre class="cxx">
using query = odb::query&lt;employee>;
using result = odb::result&lt;employee>;

session s;
transaction t (db.begin ());

result r (db.query&lt;employee> (
  query::employer->name == "Example Inc" &amp;&amp; query::last == "Doe"));

for (result::iterator i (r.begin ()); i != r.end (); ++i)
  cout &lt;&lt; i->first_ &lt;&lt; " " &lt;&lt; i->last_ &lt;&lt; endl;

t.commit ();
  </pre>

  <p>A query class member corresponding to a non-inverse
     (<a href="#6.2">Section 6.2, "Bidirectional Relationships"</a>) object
     pointer can also be used as a normal member that has the id type
     of the pointed-to object. For example, the following query locates
     all the <code>employee</code> objects that don't have an associated
     <code>employer</code> object:</p>

  <pre class="cxx">
result r (db.query&lt;employee> (query::employer.is_null ()));
  </pre>

  <p>An important concept to keep in mind when working with object
     relationships is the independence of persistent objects. In particular,
     when an object containing a pointer to another object is made persistent
     or is updated, the pointed-to object is not automatically persisted
     or updated. Rather, only a reference to the object (in the form of the
     object id) is stored for the pointed-to object in the database.
     The pointed-to object itself is a separate entity and should
     be made persistent or updated independently. By default, the
     same principle also applies to erasing pointed-to objects. That
     is, we have to make sure all the pointing objects are updated
     accordingly. However, in the case of erase, we can specify an
     alternative <code>on-delete</code> semantic as discussed in
     <a href="#14.4.15">Section 14.4.15, "<code>on_delete</code>"</a>.</p>

  <p>When persisting or updating an object containing a pointer to another
     object, the pointed-to object must have a valid object id. This,
     however, may not always be easy to achieve in complex relationships that
     involve objects with automatically assigned identifiers. In such
     cases it may be necessary to first persist an object with a pointer
     set to <code>NULL</code> and then, once the pointed-to object is
     made persistent and its identifier assigned, set the pointer
     to the correct value and update the object in the database.</p>

  <p>Persistent object relationships can be divided into two groups:
     unidirectional and bidirectional. Each group in turn contains
     several configurations that vary depending on the cardinality
     of the sides of the relationship. All possible unidirectional
     and bidirectional configurations are discussed in the following
     sections.</p>

  <h2><a name="6.1">6.1 Unidirectional Relationships</a></h2>

  <p>In unidirectional relationships we are only interested in navigating
     from object to object in one direction. Because there is no interest
     in navigating in the opposite direction, the cardinality of the other
     end of the relationship is unimportant. As a result, there are only
     two possible unidirectional relationships: to-one and to-many. Each
     of these relationships is described in the following sections. For
     sample code that shows how to work with these relationships, refer
     to the <code>relationship</code> example in the <code>odb-examples</code>
     package.</p>

  <h3><a name="6.1.1">6.1.1 To-One Relationships</a></h3>

  <p>An example of a unidirectional to-one relationship is the
     employee-employer relationship (an employee has one employer).
     The following persistent C++ classes model this relationship:</p>

  <pre class="cxx">
#pragma db object
class employer
{
  ...

  #pragma db id
  std::string name_;
};

#pragma db object
class employee
{
  ...

  #pragma db id
  unsigned long long id_;

  #pragma db not_null
  shared_ptr&lt;employer> employer_;
};
  </pre>

  <p>The corresponding database tables look like this:</p>

  <pre class="sql">
CREATE TABLE employer (
  name VARCHAR (128) NOT NULL PRIMARY KEY);

CREATE TABLE employee (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY,
  employer VARCHAR (128) NOT NULL REFERENCES employer (name));
  </pre>

  <h3><a name="6.1.2">6.1.2 To-Many Relationships</a></h3>

  <p>An example of a unidirectional to-many relationship is the
     employee-project relationship (an employee can be involved
     in multiple projects). The following persistent C++ classes
     model this relationship:</p>

  <pre class="cxx">
#pragma db object
class project
{
  ...

  #pragma db id
  std::string name_;
};

#pragma db object
class employee
{
  ...

  #pragma db id
  unsigned long long id_;

  #pragma db value_not_null unordered
  std::vector&lt;shared_ptr&lt;project>> projects_;
};
  </pre>

  <p>The corresponding database tables look like this:</p>

  <pre class="sql">
CREATE TABLE project (
  name VARCHAR (128) NOT NULL PRIMARY KEY);

CREATE TABLE employee (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY);

CREATE TABLE employee_projects (
  object_id BIGINT UNSIGNED NOT NULL,
  value VARCHAR (128) NOT NULL REFERENCES project (name));
  </pre>

  <p>To obtain a more canonical database schema, the names of tables
     and columns above can be customized using ODB pragmas
     (<a href="#14">Chapter 14, "ODB Pragma Language"</a>). For example:</p>

  <pre class="cxx">
#pragma db object
class employee
{
  ...

  #pragma db value_not_null unordered \
             id_column("employee_id") value_column("project_name")
  std::vector&lt;shared_ptr&lt;project>> projects_;
};
  </pre>

  <p>The resulting <code>employee_projects</code> table would then
     look like this:</p>

  <pre class="sql">
CREATE TABLE employee_projects (
  employee_id BIGINT UNSIGNED NOT NULL,
  project_name VARCHAR (128) NOT NULL REFERENCES project (name));
  </pre>


  <h2><a name="6.2">6.2 Bidirectional Relationships</a></h2>

  <p>In bidirectional relationships we are interested in navigating
     from object to object in both directions. As a result, each
     object class in a relationship contains a pointer to the other
     object. If smart pointers are used, then a weak pointer should
     be used as one of the pointers to avoid ownership cycles. For
     example:</p>

  <pre class="cxx">
class employee;

#pragma db object
class position
{
  ...

  #pragma db id
  unsigned long long id_;

  weak_ptr&lt;employee> employee_;
};

#pragma db object
class employee
{
  ...

  #pragma db id
  unsigned long long id_;

  #pragma db not_null
  shared_ptr&lt;position> position_;
};
  </pre>

  <p>Note that when we establish a bidirectional relationship, we
     have to set both pointers consistently. One way to make sure
     that a relationship is always in a consistent state is to
     provide a single function that updates both pointers at the
     same time. For example:</p>

  <pre class="cxx">
#pragma db object
class position: public enable_shared_from_this&lt;position>
{
  ...

  void
  fill (shared_ptr&lt;employee> e)
  {
    employee_ = e;
    e->positions_ = shared_from_this ();
  }

private:
  weak_ptr&lt;employee> employee_;
};

#pragma db object
class employee
{
  ...

private:
  friend class position;

  #pragma db not_null
  shared_ptr&lt;position> position_;
};
  </pre>

  <p>At the beginning of this chapter we examined how to use a session
     to make sure a single object is shared among all other objects pointing
     to it. With bidirectional relationships involving weak pointers the
     use of a session becomes even more crucial. Consider the following
     transaction that tries to load the <code>position</code> object
     from the above example without using a session:</p>

  <pre class="cxx">
transaction t (db.begin ())
shared_ptr&lt;position> p (db.load&lt;position> (1));
...
t.commit ();
  </pre>

  <p>When we load the <code>position</code> object, the <code>employee</code>
     object, which it points to, is also loaded. While <code>employee</code>
     is initially stored as <code>shared_ptr</code>, it is then assigned to
     the <code>employee_</code> member which is <code>weak_ptr</code>. Once
     the assignment is complete, the shared pointer goes out of scope
     and the only pointer that points to the newly loaded
     <code>employee</code> object is the <code>employee_</code> weak
     pointer. And that means the <code>employee</code> object is deleted
     immediately after being loaded. To help avoid such pathological
     situations ODB detects cases where a newly loaded object will
     immediately be deleted and throws the <code>odb::session_required</code>
     exception.</p>

  <p>As the exception name suggests, the easiest way to resolve this
     problem is to use a session:</p>

  <pre class="cxx">
session s;
transaction t (db.begin ())
shared_ptr&lt;position> p (db.load&lt;position> (1));
...
t.commit ();
  </pre>

  <p>In our example, the session will maintain a shared pointer to the
     loaded <code>employee</code> object preventing its immediate
     deletion. Another way to resolve this problem is to avoid
     immediate loading of the pointed-to objects using lazy weak
     pointers. Lazy pointers are discussed in <a href="#6.4">Section 6.4,
     "Lazy Pointers"</a> later in this chapter.</p>

  <p>Above, to model a bidirectional relationship in persistent classes,
     we used two pointers, one in each object. While this is a natural
     representation in C++, it does not translate to a canonical
     relational model. Consider the database schema generated for
     the above two classes:</p>

  <pre class="sql">
CREATE TABLE position (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY,
  employee BIGINT UNSIGNED REFERENCES employee (id));

CREATE TABLE employee (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY,
  position BIGINT UNSIGNED NOT NULL REFERENCES position (id));
  </pre>

  <p>While this database schema is valid, it is unconventional. We have
     a reference from a row in the <code>position</code> table to a row
     in the <code>employee</code> table. We also have a reference
     from this same row in the <code>employee</code> table back to
     the row in the <code>position</code> table. From the relational
     point of view, one of these references is redundant since
     in SQL we can easily navigate in both directions using just one
     of these references.</p>

  <p>To eliminate redundant database schema references we can use the
     <code>inverse</code> pragma (<a href="#14.4.14">Section 14.4.14,
     "<code>inverse</code>"</a>) which tells the ODB compiler that
     a pointer is the inverse side of a bidirectional relationship.
     Either side of a relationship can be made inverse. For example:</p>

  <pre class="cxx">
#pragma db object
class position
{
  ...

  #pragma db inverse(position_)
  weak_ptr&lt;employee> employee_;
};

#pragma db object
class employee
{
  ...

  #pragma db not_null
  shared_ptr&lt;position> position_;
};
  </pre>

  <p>The resulting database schema looks like this:</p>

  <pre class="sql">
CREATE TABLE position (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY);

CREATE TABLE employee (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY,
  position BIGINT UNSIGNED NOT NULL REFERENCES position (id));
  </pre>

  <p>As you can see, an inverse member does not have a corresponding
     column (or table, in case of an inverse container of pointers)
     and, from the point of view of database operations, is effectively
     read-only. The only way to change a bidirectional relationship
     with an inverse side is to set its direct (non-inverse)
     pointer. Also note that an ordered container (<a href="#5.1">Section
     5.1, "Ordered Containers"</a>) of pointers that is an inverse side
     of a bidirectional relationship is always treated as unordered
     (<a href="#14.4.19">Section 14.4.19, "<code>unordered</code>"</a>)
     because the contents of such a container are implicitly built from
     the direct side of the relationship which does not contain the
     element order (index).</p>

  <p>There are three distinct bidirectional relationships that we
     will cover in the following sections: one-to-one, one-to-many,
     and many-to-many. We will only talk about bidirectional
     relationships with inverse sides since they result in canonical
     database schemas. For sample code that shows how to work with
     these relationships, refer to the <code>inverse</code> example
     in the <code>odb-examples</code> package.</p>

  <h3><a name="6.2.1">6.2.1 One-to-One Relationships</a></h3>

  <p>An example of a bidirectional one-to-one relationship is the
     presented above employee-position relationship (an employee
     fills one position and a position is filled by one employee).
     The following persistent C++ classes model this relationship:</p>

  <pre class="cxx">
class employee;

#pragma db object
class position
{
  ...

  #pragma db id
  unsigned long long id_;

  #pragma db inverse(position_)
  weak_ptr&lt;employee> employee_;
};

#pragma db object
class employee
{
  ...

  #pragma db id
  unsigned long long id_;

  #pragma db not_null
  shared_ptr&lt;position> position_;
};
  </pre>

  <p>The corresponding database tables look like this:</p>

  <pre class="sql">
CREATE TABLE position (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY);

CREATE TABLE employee (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY,
  position BIGINT UNSIGNED NOT NULL REFERENCES position (id));
  </pre>

  <p>If instead the other side of this relationship is made inverse,
     then the database tables will change as follows:</p>

  <pre class="sql">
CREATE TABLE position (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY,
  employee BIGINT UNSIGNED REFERENCES employee (id));

CREATE TABLE employee (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY);
  </pre>

  <h3><a name="6.2.2">6.2.2 One-to-Many Relationships</a></h3>

  <p>An example of a bidirectional one-to-many relationship is the
     employer-employee relationship (an employer has multiple
     employees and an employee is employed by one employer).
     The following persistent C++ classes model this relationship:</p>

  <pre class="cxx">
class employee;

#pragma db object
class employer
{
  ...

  #pragma db id
  std::string name_;

  #pragma db value_not_null inverse(employer_)
  std::vector&lt;weak_ptr&lt;employee>> employees_
};

#pragma db object
class employee
{
  ...

  #pragma db id
  unsigned long long id_;

  #pragma db not_null
  shared_ptr&lt;employer> employer_;
};
  </pre>

  <p>The corresponding database tables differ significantly depending
     on which side of the relationship is made inverse. If the <em>one</em>
     side (<code>employer</code>) is inverse as in the code
     above, then the resulting database schema looks like this:</p>

  <pre class="sql">
CREATE TABLE employer (
  name VARCHAR (128) NOT NULL PRIMARY KEY);

CREATE TABLE employee (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY,
  employer VARCHAR (128) NOT NULL REFERENCES employer (name));
  </pre>

  <p>If instead the <em>many</em> side (<code>employee</code>) of this
     relationship is made inverse, then the database tables will change
     as follows:</p>

  <pre class="sql">
CREATE TABLE employer (
  name VARCHAR (128) NOT NULL PRIMARY KEY);

CREATE TABLE employer_employees (
  object_id VARCHAR (128) NOT NULL REFERENCES employer (name),
  value BIGINT UNSIGNED NOT NULL REFERENCES employee (id));

CREATE TABLE employee (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY);
  </pre>

  <h3><a name="6.2.3">6.2.3 Many-to-Many Relationships</a></h3>

  <p>An example of a bidirectional many-to-many relationship is the
     employee-project relationship (an employee can work on multiple
     projects and a project can have multiple participating employees).
     The following persistent C++ classes model this relationship:</p>

  <pre class="cxx">
class employee;

#pragma db object
class project
{
  ...

  #pragma db id
  std::string name_;

  #pragma db value_not_null inverse(projects_)
  std::vector&lt;weak_ptr&lt;employee>> employees_;
};

#pragma db object
class employee
{
  ...

  #pragma db id
  unsigned long long id_;

  #pragma db value_not_null unordered
  std::vector&lt;shared_ptr&lt;project>> projects_;
};
  </pre>

  <p>The corresponding database tables look like this:</p>

  <pre class="sql">
CREATE TABLE project (
  name VARCHAR (128) NOT NULL PRIMARY KEY);

CREATE TABLE employee (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY);

CREATE TABLE employee_projects (
  object_id BIGINT UNSIGNED NOT NULL REFERENCES employee (id),
  value VARCHAR (128) NOT NULL REFERENCES project (name));
  </pre>

  <p>If instead the other side of this relationship is made inverse,
     then the database tables will change as follows:</p>

  <pre class="sql">
CREATE TABLE project (
  name VARCHAR (128) NOT NULL PRIMARY KEY);

CREATE TABLE project_employees (
  object_id VARCHAR (128) NOT NULL REFERENCES project (name),
  value BIGINT UNSIGNED NOT NULL REFERENCES employee (id));

CREATE TABLE employee (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY);
  </pre>

  <h2><a name="6.3">6.3 Circular Relationships</a></h2>

  <p>A relationship between two persistent classes is circular if each
     of them references the other. Bidirectional relationships are
     always circular. A unidirectional relationship combined with
     inheritance (<a href="#8">Chapter 8, "Inheritance"</a>) can also
     be circular. For example, the <code>employee</code> class could
     derive from <code>person</code> which, in turn, could contain a
     pointer to <code>employee</code>.</p>

  <p>We don't need to do anything extra if persistent classes with
     circular dependencies are defined in the same header
     file. Specifically, ODB will make sure that the database tables
     and foreign key constraints are created in the correct order. As a
     result, unless you have good reasons not to, it is recommended that
     you keep persistent classes with circular dependencies in the same
     header file.</p>

  <p>If you have to keep such classes in separate header files, then
     there are two extra steps that you may need to take in order to
     use these classes with ODB. Consider again the example from
     <a href="#6.2.1">Section 6.2.1, "One-to-One Relationships"</a>
     but this time with the classes defined in separate headers:</p>

  <pre class="cxx">
// position.hxx
//
class employee;

#pragma db object
class position
{
  ...

  #pragma db id
  unsigned long long id_;

  #pragma db inverse(position_)
  weak_ptr&lt;employee> employee_;
};
  </pre>

  <pre class="cxx">
// employee.hxx
//
#include "position.hxx"

#pragma db object
class employee
{
  ...

  #pragma db id
  unsigned long long id_;

  #pragma db not_null
  shared_ptr&lt;position> position_;
};
  </pre>

  <p>Note that the <code>position.hxx</code> header contains only the forward
     declaration for <code>employee</code>. While this is sufficient to
     define a valid, from the C++ point of view, <code>position</code> class,
     the ODB compiler needs to "see" the definitions of the pointed-to
     persistent classes. There are several ways we can fulfil this
     requirement. The easiest is to simply include <code>employee.hxx</code>
     at the end of <code>position.hxx</code>:</p>

  <pre class="cxx">
// position.hxx
//
class employee;

#pragma db object
class position
{
  ...
};

#include "employee.hxx"
  </pre>

  <p>We can also limit this inclusion only to the time when
     <code>position.hxx</code> is compiled with the ODB compiler:</p>

  <pre class="cxx">
// position.hxx
//

...

#ifdef ODB_COMPILER
#  include "employee.hxx"
#endif
  </pre>

  <p>Finally, if we don't want to modify <code>position.hxx</code>,
     then we can add <code>employee.hxx</code> to the ODB compilation
     process with the <code>--odb-epilogue</code> option. For example:</p>

  <pre class="terminal">
odb ... --odb-epilogue "#include \"employee.hxx\"" position.hxx
  </pre>

  <p>Note also that in this example we didn't have to do anything extra
     for <code>employee.hxx</code> because it already includes
     <code>position.hxx</code>. However, if instead it relied only
     on the forward declaration of the <code>position</code> class,
     then we would have to handle it in the same way as
     <code>position.hxx</code>.</p>

  <p>The other difficulty with separately defined classes involving
     circular relationships has to do with the correct order of foreign
     key constraint creation in the generated database schema. In
     the above example, if we generate the database schema as
     standalone SQL files, then we will end up with two such files:
     <code>position.sql</code> and <code>employee.sql</code>.
     If we try to execute <code>employee.sql</code> first, then
     we will get an error indicating that the table corresponding to
     the <code>position</code> class and referenced by the foreign
     key constraint corresponding to the <code>position_</code>
     pointer does not yet exist.</p>

  <p>Note that there is no such problem if the database schema
     is embedded in the generated C++ code instead of being produced
     as standalone SQL files. In this case, the ODB compiler is
     able to ensure the correct creation order even if the classes
     are defined in separate header files.</p>

  <p>In certain cases, for example, a bidirectional relationship
     with an inverse side, this problem can be resolved by executing
     the database schema creation files in the correct order. In our
     example, this would be <code>position.sql</code> first
     and <code>employee.sql</code> second. However, this approach
     doesn't scale beyond simple object models.</p>

  <p>A more robust solution to this problem is to generate the database
     schema for all the persistent classes into a single SQL file. This
     way, the ODB compiler can again ensure the correct creation order
     of tables and foreign keys. To instruct the ODB compiler to produce
     a combined schema file for several headers we can use the
     <code>--generate-schema-only</code> and <code>--at-once</code>
     options. For example:</p>

  <pre class="terminal">
odb ... --generate-schema-only --at-once --input-name company \
position.hxx employee.hxx
  </pre>

  <p>The result of the above command is a single <code>company.sql</code>
     file (the name is derived from the <code>--input-name</code> value)
     that contains the database creation code for both <code>position</code>
     and <code>employee</code> classes.</p>

  <h2><a name="6.4">6.4 Lazy Pointers</a></h2>

  <p>Consider again the bidirectional, one-to-many employer-employee
     relationship that was presented earlier in this chapter:</p>

  <pre class="cxx">
class employee;

#pragma db object
class employer
{
  ...

  #pragma db id
  std::string name_;

  #pragma db value_not_null inverse(employer_)
  std::vector&lt;weak_ptr&lt;employee>> employees_;
};

#pragma db object
class employee
{
  ...

  #pragma db id
  unsigned long long id_;

  #pragma db not_null
  shared_ptr&lt;employer> employer_;
};
  </pre>

  <p>Consider also the following transaction which obtains the employer
     name given the employee id:</p>

  <pre class="cxx">
unsigned long long id = ...
string name;

session s;
transaction t (db.begin ());

shared_ptr&lt;employee> e (db.load&lt;employee> (id));
name = e->employer_->name_;

t.commit ();
  </pre>

  <p>While this transaction looks very simple, it actually does a lot more
     than what meets the eye and is necessary. Consider what happens when
     we load the <code>employee</code> object: the <code>employer_</code>
     pointer is also automatically loaded which means the <code>employer</code>
     object corresponding to this employee is also loaded. But the
     <code>employer</code> object in turn contains the list of pointers
     to all the employees, which are also loaded. As a result, when object
     relationships are involved, a simple transaction like the above can
     load many more objects than is necessary.</p>

  <p>To overcome this problem ODB offers finer grained control over
     the relationship loading in the form of lazy pointers. A lazy
     pointer does not automatically load the pointed-to object
     when the containing object is loaded. Instead, we have to
     explicitly load the pointed-to object if and when we need to
     access it.</p>

  <p>The ODB runtime library provides lazy counterparts for all the
     supported pointers, namely:
     <code>odb::lazy_shared_ptr</code>/<code>lazy_weak_ptr</code>
     for C++11 <code>std::shared_ptr</code>/<code>weak_ptr</code>,
     <code>odb::lazy_unique_ptr</code> for C++11 <code>std::unique_ptr</code>,
     <code>odb::lazy_auto_ptr</code> for C++98/03 <code>std::auto_ptr</code>,
     and <code>odb::lazy_ptr</code> for raw pointers. All these lazy pointers
     are defined in the <code>&lt;odb/lazy-ptr.hxx></code> header. The ODB
     profile libraries also provide lazy pointer implementations for smart
     pointers from popular frameworks and libraries (<a href="#III">Part III,
     "Profiles"</a>).</p>

  <p>While we will discuss the interface of lazy pointers in more detail
     shortly, the most commonly used extra function provided by these
     pointers is <code>load()</code>. This function loads the
     pointed-to object if it hasn't already been loaded. After
     the call to this function, the lazy pointer can be used
     in the the same way as its eager counterpart. The <code>load()</code>
     function also returns the eager pointer, in case you need to pass
     it around. For a lazy weak pointer, the
     <code>load()</code> function also locks the pointer.</p>

  <p>The following example shows how we can change our employer-employee
     relationship to use lazy pointers. Here we choose to use lazy pointers
     for both sides of the relationship.</p>

  <pre class="cxx">
class employee;

#pragma db object
class employer
{
  ...

  #pragma db value_not_null inverse(employer_)
  std::vector&lt;lazy_weak_ptr&lt;employee>> employees_;
};

#pragma db object
class employee
{
  ...

  #pragma db not_null
  lazy_shared_ptr&lt;employer> employer_;
};
  </pre>

  <p>And the transaction is changed like this:</p>

  <pre class="cxx">
unsigned long long id = ...
string name;

session s;
transaction t (db.begin ());

shared_ptr&lt;employee> e (db.load&lt;employee> (id));
e->employer_.load ();
name = e->employer_->name_;

t.commit ();
  </pre>


  <p>As a general guideline we recommend that you make at least one side
     of a bidirectional relationship lazy, especially for relationships
     with a <em>many</em> side.</p>

  <p>A lazy pointer implementation mimics the interface of its eager
     counterpart which can be used once the pointer is loaded. It also
     adds a number of additional functions that are specific to the
     lazy loading functionality. Overall, the interface of a lazy
     pointer follows this general outline:</p>

  <pre class="cxx">
template &lt;class T>
class lazy_ptr
{
public:
  //
  // The eager pointer interface.
  //

  // Initialization/assignment from an eager pointer to a
  // transient object.
  //
public:
  template &lt;class Y> lazy_ptr (const eager_ptr&lt;Y>&amp;);
  template &lt;class Y> lazy_ptr&amp; operator= (const eager_ptr&lt;Y>&amp;);

  // Lazy loading interface.
  //
public:
  //  NULL      loaded()
  //
  //  true       true      NULL pointer to transient object
  //  false      true      valid pointer to persistent object
  //  true       false     unloaded pointer to persistent object
  //  false      false     valid pointer to transient object
  //
  bool loaded () const;

  eager_ptr&lt;T> load () const;

  // Unload the pointer. For transient objects this function is
  // equivalent to reset().
  //
  void unload () const;

  // Get the underlying eager pointer. If this is an unloaded pointer
  // to a persistent object, then the returned pointer will be NULL.
  //
  eager_ptr&lt;T> get_eager () const;

  // Initialization with a persistent loaded object.
  //
  template &lt;class Y> lazy_ptr (database&amp;, Y*);
  template &lt;class Y> lazy_ptr (database&amp;, const eager_ptr&lt;Y>&amp;);

  template &lt;class Y> void reset (database&amp;, Y*);
  template &lt;class Y> void reset (database&amp;, const eager_ptr&lt;Y>&amp;);

  // Initialization with a persistent unloaded object.
  //
  template &lt;class ID> lazy_ptr (database&amp;, const ID&amp;);

  template &lt;class ID> void reset (database&amp;, const ID&amp;);

  // Query object id and database of a persistent object.
  //
  template &lt;class O /* = T */>
  // C++11: template &lt;class O = T>
  object_traits&lt;O>::id_type object_id () const;

  odb::database&amp; database () const;
};
  </pre>

  <p>Note that to initialize a lazy pointer to a persistent object from
     its eager pointer one must use the constructor or <code>reset()</code>
     function with the database as its first argument. The database is
     required to support comparison of unloaded lazy pointers to persistent
     objects.</p>

  <p>In a lazy weak pointer interface, the <code>load()</code> function
     returns the <em>strong</em> (shared) eager pointer. The following
     transaction demonstrates the use of a lazy weak pointer based on
     the <code>employer</code> and <code>employee</code> classes
     presented earlier.</p>

  <pre class="cxx">
using employees = std::vector&lt;lazy_weak_ptr&lt;employee>>;

session s;
transaction t (db.begin ());

shared_ptr&lt;employer> er (db.load&lt;employer> ("Example Inc"));
employees&amp; es (er->employees ());

for (employees::iterator i (es.begin ()); i != es.end (); ++i)
{
  // We are only interested in employees with object id less than
  // 100.
  //
  lazy_weak_ptr&lt;employee>&amp; lwp (*i);

  if (lwp.object_id&lt;employee> () &lt; 100)
  // C++11: if (lwp.object_id () &lt; 100)
  {
    shared_ptr&lt;employee> e (lwp.load ()); // Load and lock.
    cout &lt;&lt; e->first_ &lt;&lt; " " &lt;&lt; e->last_ &lt;&lt; endl;
  }
}

t.commit ();
  </pre>

  <p>Notice that inside the for-loop we use a reference to the lazy
     weak pointer instead of making a copy. This is not merely to
     avoid a copy. When a lazy pointer is loaded, all other lazy
     pointers that point to the same object do not automatically
     become loaded (though an attempt to load such copies will
     result in them pointing to the same object, provided the
     same session is still in effect). By using a reference
     in the above transaction we make sure that we load the
     pointer that is contained in the <code>employer</code>
     object. This way, if we later need to re-examine this
     <code>employee</code> object, the pointer will already
     be loaded.</p>

  <p>As another example, suppose we want to add an employee
     to Example Inc. The straightforward implementation of this
     transaction is presented below:</p>

  <pre class="cxx">
session s;
transaction t (db.begin ());

shared_ptr&lt;employer> er (db.load&lt;employer> ("Example Inc"));
shared_ptr&lt;employee> e (new employee ("John", "Doe"));

e->employer_ = er;
er->employees ().push_back (e);

db.persist (e);
t.commit ();
  </pre>

  <p>Notice here that we didn't have to update the employer object
     in the database since the <code>employees_</code> list of
     pointers is an inverse side of a bidirectional relationship
     and is effectively read-only, from the persistence point of
     view.</p>

  <p>A faster implementation of this transaction, that avoids loading
     the employer object, relies on the ability to initialize an
     <em>unloaded</em> lazy pointer with the database where the object
     is stored as well as its identifier:</p>

  <pre class="cxx">
lazy_shared_ptr&lt;employer> er (db, std::string ("Example Inc"));
shared_ptr&lt;employee> e (new employee ("John", "Doe"));

e->employer_ = er;

session s;
transaction t (db.begin ());

db.persist (e);

t.commit ();
  </pre>

  <p>For the interaction of lazy pointers with lazy-loaded object
     sections, refer to <a href="#9.3">Section 9.3, "Sections and
     Lazy Pointers"</a>.</p>

  <h2><a name="6.5">6.5 Using Custom Smart Pointers</a></h2>

  <p>While the ODB runtime and profile libraries provide support for
     the majority of widely-used pointers, it is also easy to add
     support for a custom smart pointer.</p>

  <p>To achieve this you will need to implement the
     <code>pointer_traits</code> class template specialization for
     your pointer. The first step is to determine the pointer kind
     since the interface of the <code>pointer_traits</code> specialization
     varies depending on the pointer kind. The supported pointer kinds
     are: <em>raw</em> (raw pointer or equivalent, that is, unmanaged),
          <em>unique</em> (smart pointer that doesn't support sharing),
          <em>shared</em> (smart pointer that supports sharing), and
          <em>weak</em> (weak counterpart of the shared pointer). Any of
     these pointers can be lazy, which also affects the
     interface of the <code>pointer_traits</code> specialization.</p>

  <p>Once you have determined the pointer kind for your smart pointer,
     use a specialization for one of the standard pointers found in
     the common ODB runtime library (<code>libodb</code>) as a base
     for your own implementation.</p>

  <p>Once the pointer traits specialization is ready, you will need to
     include it into the ODB compilation process using the
     <code>--odb-epilogue</code> option and into the generated header
     files with the <code>--hxx-prologue</code> option. As an example,
     suppose we have the <code>smart_ptr</code> smart pointer for which
     we have the traits specialization implemented in the
     <code>smart-ptr-traits.hxx</code> file. Then, we can create an ODB
     compiler options file for this pointer and save it to
     <code>smart-ptr.options</code>:</p>

  <pre>
# Options file for smart_ptr.
#
--odb-epilogue '#include "smart-ptr-traits.hxx"'
--hxx-prologue '#include "smart-ptr-traits.hxx"'
  </pre>

  <p>Now, whenever we compile a header file that uses <code>smart_ptr</code>,
     we can specify the following command line option to make sure it is
     recognized by the ODB compiler as a smart pointer and the traits file
     is included in the generated code:</p>

  <pre>
--options-file smart-ptr.options
  </pre>

  <p>It is also possible to implement a lazy counterpart for your
     smart pointer. The ODB runtime library provides a class template
     that encapsulates the object id management and loading
     functionality that is needed to implement a lazy pointer. All
     you need to do is wrap it with an interface that mimics
     your smart pointer. Using one of the existing lazy pointer
     implementations (either from the ODB runtime library or one
     of the profile libraries) as a base for your implementation
     is the easiest way to get started.</p>


  <!-- CHAPTER -->

  <hr class="page-break"/>
  <h1><a name="7">7 Value Types</a></h1>

  <p>In <a href="#3.1">Section 3.1, "Concepts and Terminology"</a> we have
     already discussed the notion of values and value types as well as the
     distinction between simple and composite values. This chapter covers
     simple and composite value types in more detail.</p>

  <h2><a name="7.1">7.1 Simple Value Types</a></h2>

  <p>A simple value type is a fundamental C++ type or a class type that
     is mapped to a single database column. For each supported database
     system the ODB compiler provides a default mapping to suitable
     database types for most fundamental C++ types, such as <code>int</code>
     or <code>float</code> as well as some class types, such as
     <code>std::string</code>. For more information about the default
     mapping for each database system refer to <a href="#II">Part II,
     Database Systems</a>. We can also provide a custom mapping for
     these or our own value types using the <code>db&nbsp;type</code>
     pragma (<a href="#14.3.1">Section 14.3.1, "<code>type</code>"</a>).</p>

  <h2><a name="7.2">7.2 Composite Value Types</a></h2>

  <p>A composite value type is a <code>class</code> or <code>struct</code>
     type that is mapped to more than one database column. To declare
     a composite value type we use the <code>db&nbsp;value</code> pragma,
     for example:</p>

  <pre class="cxx">
#pragma db value
class basic_name
{
  ...

  std::string first_;
  std::string last_;
};
  </pre>

  <p>The complete version of the above code fragment and the other code
     samples presented in this section can be found in the <code>composite</code>
     example in the <code>odb-examples</code> package.</p>

  <p>A composite value type does not have to define a default constructor,
     unless it is used as an element of a container. In this case the
     default constructor can be made private provided we also make the
     <code>odb::access</code> class, defined in the
     <code>&lt;odb/core.hxx></code> header, a friend of this value type.
     For example:</p>

  <pre class="cxx">
#include &lt;odb/core.hxx>

#pragma db value
class basic_name
{
public:
  basic_name (const std::string&amp; first, const std::string&amp; last);

  ...

private:
  friend class odb::access;

  basic_name () {} // Needed for storing basic_name in containers.

  ...
};
  </pre>

  <p>The ODB compiler also needs access to the non-transient
     (<a href="#14.4.11">Section 14.4.11, "<code>transient</code>"</a>)
     data members of a composite value type. It uses the same mechanisms
     as for persistent classes which are discussed in
     <a href="#3.2">Section 3.2, "Declaring Persistent Objects and
     Values"</a>.</p>

  <p>The members of a composite value can be other value types (either
     simple or composite), containers (<a href="#5">Chapter 5,
     "Containers"</a>), and pointers to objects (<a href="#6">Chapter 6,
     "Relationships"</a>).
     Similarly, a composite value type can be used in object members,
     as an element of a container, and as a base for another composite
     value type. In particular, composite value types can be used as
     element types in set containers (<a href="#5.2">Section 5.2, "Set
     and Multiset Containers"</a>) and as key types in map containers
     (<a href="#5.3">Section 5.3, "Map and Multimap Containers"</a>).
     A composite value type that is used as an element of a container
     cannot contain other containers since containers of containers
     are not allowed. The following example illustrates some of the
     possible use cases:</p>

  <pre class="cxx">
#pragma db value
class basic_name
{
  ...

  std::string first_;
  std::string last_;
};

using basic_names = std::vector&lt;basic_name>;

#pragma db value
class name_extras
{
  ...

  std::string nickname_;
  basic_names aliases_;
};

#pragma db value
class name: public basic_name
{
  ...

  std::string title_;
  name_extras extras_;
};

#pragma db object
class person
{
  ...

  name name_;
};
  </pre>

  <p>A composite value type can be defined inside a persistent class,
     view, or another composite value and even made private, provided
     we make <code>odb::access</code> a friend of the containing class,
     for example:</p>

<pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db value
  class name
  {
    ...

    std::string first_;
    std::string last_;
  };

  name name_;
};
  </pre>

  <p>A composite value type can also be defined as an instantiation
     of a C++ class template, for example:</p>

  <pre class="cxx">
template &lt;typename T>
struct point
{
  T x;
  T y;
  T z;
};

using int_point = point&lt;int>;
#pragma db value(int_point)

#pragma db object
class object
{
  ...

  int_point center_;
};
  </pre>

  <p>Note that the database support code for such a composite value type
     is generated when compiling the header containing the
     <code>db&nbsp;value</code> pragma and not the header containing
     the template definition or the <code>using</code> alias. This
     allows us to use templates defined in other files, such as
     <code>std::pair</code> defined in the <code>utility</code>
     standard header file:</p>

  <pre class="cxx">
#include &lt;utility> // std::pair

using phone_numbers = std::pair&lt;std::string, std::string>;
#pragma db value(phone_numbers)

#pragma db object
class person
{
  ...

  phone_numbers phone_;
};
  </pre>

  <p>We can also use data members from composite value types
     in database queries (<a href="#4">Chapter 4, "Querying the
     Database"</a>). For each composite value in a persistent class, the
     query class defines a nested member that contains members corresponding
     to the data members in the value type. We can then use the member access
     syntax (.) to refer to data members in value types. For example, the
     query class for the <code>person</code> object presented above
     contains the <code>name</code> member (its name is derived from
     the <code>name_</code> data member) which in turn contains the
     <code>extras</code> member (its name is derived from the
     <code>name::extras_</code> data member of the composite value type).
     This process continues recursively for nested composite value types
     and, as a result, we can use the <code>query::name.extras.nickname</code>
     expression while querying the database for the <code>person</code>
     objects. For example:</p>

  <pre class="cxx">
using query = odb::query&lt;person>;
using result = odb::result&lt;person>;

transaction t (db.begin ());

result r (db.query&lt;person> (
  query::name.extras.nickname == "Squeaky"));

...

t.commit ();
  </pre>

  <h3><a name="7.2.1">7.2.1 Composite Object Ids</a></h3>

  <p>An object id can be of a composite value type, for example:</p>

  <pre class="cxx">
#pragma db value
class name
{
  ...

  std::string first_;
  std::string last_;
};

#pragma db object
class person
{
  ...

  #pragma db id
  name name_;
};
  </pre>

  <p>However, a value type that can be used as an object id has a number
     of restrictions. Such a value type cannot have container, object
     pointer, or read-only data members. It also must be
     default-constructible, copy-constructible, and copy-assignable.
     Furthermore, if the persistent class in which
     this composite value type is used as object id has session support
     enabled (<a href="#11">Chapter 11, "Session"</a>), then it must also
     implement the less-than comparison operator (<code>operator&lt;</code>).</p>

  <h3><a name="7.2.2">7.2.2 Composite Value Column and Table Names</a></h3>

  <p>Customizing a column name for a data member of a simple value
     type is straightforward: we simply specify the desired name with
     the <code>db&nbsp;column</code> pragma (<a href="#14.4.9">Section
     14.4.9, "<code>column</code>"</a>). For composite value
     types things are slightly more complex since they are mapped to
     multiple columns. Consider the following example:</p>

  <pre class="cxx">
#pragma db value
class name
{
  ...

  std::string first_;
  std::string last_;
};

#pragma db object
class person
{
  ...

  #pragma db id auto
  unsigned long long id_;

  name name_;
};
  </pre>

  <p>The column names for the <code>first_</code> and <code>last_</code>
     members are constructed by using the sanitized name of the
     <code>person::name_</code> member as a prefix and the names of the
     members in the value type (<code>first_</code> and <code>last_</code>)
     as suffixes. As a result, the database schema for the above classes
     will look like this:</p>

  <pre class="sql">
CREATE TABLE person (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY,
  name_first TEXT NOT NULL,
  name_last TEXT NOT NULL);
  </pre>

 <p>We can customize both the prefix and the suffix using the
    <code>db&nbsp;column</code> pragma as shown in the following
    example:</p>

  <pre class="cxx">
#pragma db value
class name
{
  ...

  #pragma db column("first_name")
  std::string first_;

  #pragma db column("last_name")
  std::string last_;
};

#pragma db object
class person
{
  ...

  #pragma db column("person_")
  name name_;
};
  </pre>

  <p>The database schema changes as follows:</p>

  <pre class="sql">
CREATE TABLE person (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY,
  person_first_name TEXT NOT NULL,
  person_last_name TEXT NOT NULL);
  </pre>

  <p>We can also make the column prefix empty, for example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db column("")
  name name_;
};
  </pre>

  <p>This will result in the following schema:</p>

  <pre class="sql">
CREATE TABLE person (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY,
  first_name TEXT NOT NULL,
  last_name TEXT NOT NULL);
  </pre>

  <p>The same principle applies when a composite value type is used
     as an element of a container, except that instead of
     <code>db&nbsp;column</code>, either the <code>db&nbsp;value_column</code>
     (<a href="#14.4.36">Section 14.4.36, "<code>value_column</code>"</a>) or
     <code>db&nbsp;key_column</code>
     (<a href="#14.4.35">Section 14.4.35, "<code>key_column</code>"</a>)
     pragmas are used to specify the column prefix.</p>

  <p>When a composite value type contains a container, an extra table
     is used to store its elements (<a href="#5">Chapter 5, "Containers"</a>).
     The names of such tables are constructed in a way similar to the
     column names, except that by default both the object name and the
     member name are used as a prefix. For example:</p>

  <pre class="cxx">
#pragma db value
class name
{
  ...

  std::string first_;
  std::string last_;
  std::vector&lt;std::string> nicknames_;
};

#pragma db object
class person
{
  ...

  name name_;
};
  </pre>

  <p>The corresponding database schema will look like this:</p>

  <pre class="sql">
CREATE TABLE person_name_nicknames (
  object_id BIGINT UNSIGNED NOT NULL,
  index BIGINT UNSIGNED NOT NULL,
  value TEXT NOT NULL)

CREATE TABLE person (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY,
  name_first TEXT NOT NULL,
  name_last TEXT NOT NULL);
  </pre>

  <p>To customize the container table name we can use the
     <code>db&nbsp;table</code> pragma (<a href="#14.4.20">Section
     14.4.20, "<code>table</code>"</a>), for example:</p>

  <pre class="cxx">
#pragma db value
class name
{
  ...

  #pragma db table("nickname")
  std::vector&lt;std::string> nicknames_;
};

#pragma db object
class person
{
  ...

  #pragma db table("person_")
  name name_;
};
  </pre>

  <p>This will result in the following schema changes:</p>

  <pre class="sql">
CREATE TABLE person_nickname (
  object_id BIGINT UNSIGNED NOT NULL,
  index BIGINT UNSIGNED NOT NULL,
  value TEXT NOT NULL)
  </pre>

  <p>Similar to columns, we can make the table prefix empty.</p>


  <h2><a name="7.3">7.3 Pointers and <code>NULL</code> Value Semantics</a></h2>

  <p>Relational database systems have a notion of the special
     <code>NULL</code> value that is used to indicate the absence
     of a valid value in a column. While by default ODB maps
     values to columns that do not allow <code>NULL</code> values,
     it is possible to change that with the <code>db&nbsp;null</code>
     pragma (<a href="#14.4.6">Section 14.4.6,
     "<code>null</code>/<code>not_null</code>"</a>).</p>

  <p>To properly support the <code>NULL</code> semantics, the
     C++ value type must have a notion of a <code>NULL</code>
     value or a similar special state concept. Most basic
     C++ types, such as <code>int</code> or <code>std::string</code>,
     do not have this notion and therefore cannot be used directly
     for <code>NULL</code>-enabled data members (in the case of a
     <code>NULL</code> value being loaded from the database,
     such data members will be default-initialized).</p>

  <p>To allow the easy conversion of value types that do not support
     the <code>NULL</code> semantics into the ones that do, ODB
     provides the <code>odb::nullable</code> class template. It
     allows us to wrap an existing C++ type into a container-like
     class that can either be <code>NULL</code> or contain a
     value of the wrapped type. ODB also automatically enables
     the <code>NULL</code> values for data members of the
     <code>odb::nullable</code> type. For example:</p>

  <pre class="cxx">
#include &lt;odb/nullable.hxx>

#pragma db object
class person
{
  ...

  std::string first_;                    // TEXT NOT NULL
  odb::nullable&lt;std::string> middle_;    // TEXT NULL
  std::string last_;                     // TEXT NOT NULL
};
  </pre>

  <p>The <code>odb::nullable</code> class template is defined
     in the <code>&lt;odb/nullable.hxx></code> header file and
     has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  template &lt;typename T>
  class nullable
  {
  public:
    using value_type = T;

    nullable ();
    nullable (const T&amp;);
    nullable (const nullable&amp;);
    template &lt;typename Y> explicit nullable (const nullable&lt;Y>&amp;);

    nullable&amp; operator= (const T&amp;);
    nullable&amp; operator= (const nullable&amp;);
    template &lt;typename Y> nullable&amp; operator= (const nullable&lt;Y>&amp;);

    void swap (nullable&amp;);

    // Accessor interface.
    //
    bool null () const;

    T&amp;       get ();
    const T&amp; get () const;

    // Pointer interface.
    //
    operator bool_convertible () const;

    T*       operator-> ();
    const T* operator-> () const;

    T&amp;       operator* ();
    const T&amp; operator* () const;

    // Reset to the NULL state.
    //
    void reset ();
  };
}
  </pre>

  <p>The following example shows how we can use this interface:</p>

  <pre class="cxx">
  nullable&lt;string> ns;

  // Using the accessor interface.
  //
  if (ns.null ())
  {
    s = "abc";
  }
  else
  {
    string s (ns.get ());
    ns.reset ();
  }

  // The same using the pointer interface.
  //
  if (!ns)
  {
    s = "abc";
  }
  else
  {
    string s (*ns);
    ns.reset ();
  }
  </pre>


  <p>The <code>odb::nullable</code> class template requires the wrapped
     type to have public default and copy constructors as well as the
     copy assignment operator. Note also that the <code>odb::nullable</code>
     implementation is not the most efficient in that it always contains
     a fully constructed value of the wrapped type. This is normally
     not a concern for simple types such as the C++ fundamental
     types or <code>std::string</code>. However, it may become
     an issue for more complex types. In such cases you may want to
     consider using a more efficient implementation of the
     <em>optional value</em> concept such as the
     <code>optional</code> class template from Boost
     (<a href="#23.4">Section 23.4, "Optional Library"</a>).</p>

  <p>Another common C++ representation of a value that can be
     <code>NULL</code> is a pointer. ODB will automatically
     handle data members that are pointers to values, however,
     it will not automatically enable <code>NULL</code> values
     for such data members, as is the case for <code>odb::nullable</code>.
     Instead, if the <code>NULL</code> value is desired, we will
     need to enable it explicitly using the <code>db&nbsp;null</code>
     pragma. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  std::string first_;

  #pragma db null
  std::unique_ptr&lt;std::string> middle_;

  std::string last_;
};
  </pre>

  <p>The ODB compiler includes built-in support for using
     <code>std::auto_ptr</code> (C++98/03 only), <code>std::unique_ptr</code>
     (C++11), and <code>std::shared_ptr</code> (C++11) as pointers to values.
     Plus, ODB profile libraries, that are available for commonly used
     frameworks and libraries (such as Boost and Qt), provide support for
     smart pointers found in these frameworks and libraries
     (<a href="#III">Part III, "Profiles"</a>).</p>

  <p>ODB also supports the <code>NULL</code> semantics for composite
     values. In the relational database the <code>NULL</code> composite
     value is translated to <code>NULL</code> values for all the simple
     data members of this composite value. For example:</p>

  <pre class="cxx">
#pragma db value
struct name
{
  std::string first_;
  odb::nullable&lt;std::string> middle_;
  std::string last_;
};

#pragma db object
class person
{
  ...
  odb::nullable&lt;name> name_;
};
  </pre>

  <p>ODB does not support the <code>NULL</code> semantics for containers.
     This also means that a composite value that contains a container
     cannot be <code>NULL</code>. With this limitation in mind, we can
     still use smart pointers in data members of container types. The
     only restriction is that these pointers must not be <code>NULL</code>.
     For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  std::unique_ptr&lt;std::vector&lt;std::string>> aliases_;
};
  </pre>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="8">8 Inheritance</a></h1>

  <p>In C++ inheritance can be used to achieve two different goals.
     We can employ inheritance to reuse common data and functionality
     in multiple classes. For example:</p>

  <pre class="cxx">
class person
{
public:
  const std::string&amp; first () const;
  const std::string&amp; last () const;

private:
  std::string first_;
  std::string last_;
};

class employee: public person
{
  ...
};

class contractor: public person
{
  ...
};
  </pre>

 <p>In the above example both the <code>employee</code> and
    <code>contractor</code> classes inherit the <code>first_</code>
    and <code>last_</code> data members as well as the <code>first()</code>
    and <code>last()</code> accessors from the <code>person</code> base
    class.</p>

 <p>A common trait of this inheritance style, referred to as <em>reuse
    inheritance</em> from now on, is the lack of virtual functions and
    a virtual destructor in the base class. Also with this style the
    application code is normally written in terms of the derived classes
    instead of the base.</p>

 <p>The second way to utilize inheritance in C++ is to provide polymorphic
    behavior through a common interface. In this case the base class
    defines a number of virtual functions and, normally, a virtual
    destructor while the derived classes provide specific
    implementations of these virtual functions. For example:</p>

  <pre class="cxx">
class person
{
public:
  enum employment_status
  {
    unemployed,
    temporary,
    permanent,
    self_employed
  };

  virtual employment_status
  employment () const = 0;

  virtual
  ~person ();
};

class employee: public person
{
public:
  virtual employment_status
  employment () const
  {
    return temporary_ ? temporary : permanent;
  }

private:
  bool temporary_;
};

class contractor: public person
{
public:
  virtual employment_status
  employment () const
  {
    return self_employed;
  }
};
  </pre>

  <p>With this inheritance style, which we will call <em>polymorphism
     inheritance</em>, the application code normally works with derived
     classes via the base class interface. Note also that it is very common
     to mix both styles in the same hierarchy. For example, the above two
     code fragments can be combined so that the <code>person</code> base
     class provides the common data members and functions as well as
     defines the polymorphic interface.</p>

  <p>The following sections describe the available strategies for
     mapping reuse and polymorphism inheritance styles to a relational
     data model. Note also that the distinction between the two styles is
     conceptual rather than formal. For example, it is possible to treat
     a class hierarchy that defines virtual functions as a case of reuse
     inheritance if this results in the desired database mapping and
     semantics.</p>

  <p>Generally, classes that employ reuse inheritance are mapped to
     completely independent entities in the database. They use different
     object id spaces and should always be passed to and returned from
     the database operations as pointers or references to derived types.
     In other words, from the persistence point of view, such classes
     behave as if the data members from the base classes were copied
     verbatim into the derived ones.</p>

  <p>In contrast, classes that employ polymorphism inheritance share
     the object id space and can be passed to and returned from the
     database operations <em>polymorphically</em> as pointers or
     references to the base class.</p>

  <p>For both inheritance styles it is sometimes desirable to prevent
     instances of a base class from being stored in the database.
     To achieve this a persistent
     class can be declared abstract using the <code>db&nbsp;abstract</code>
     pragma (<a href="#14.1.3">Section 14.1.3, "<code>abstract</code>"</a>).
     Note that a <em>C++-abstract</em> class, or a class that
     has one or more pure virtual functions and therefore cannot be
     instantiated, is also <em>database-abstract</em>. However, a
     database-abstract class is not necessarily C++-abstract. The
     ODB compiler automatically treats C++-abstract classes as
     database-abstract.</p>

  <h2><a name="8.1">8.1 Reuse Inheritance</a></h2>

  <p>Each non-abstract class from the reuse inheritance hierarchy is
     mapped to a separate database table that contains all its data
     members, including those inherited from base classes. An abstract
     persistent class does not have to define an object id, nor a default
     constructor, and it does not have a corresponding database table.
     An abstract class cannot be a pointed-to object in a relationship.
     Multiple inheritance is supported as long as each base
     class is only inherited once. The following example shows a
     persistent class hierarchy employing reuse inheritance:</p>

  <pre class="cxx">
// Abstract person class. Note that it does not declare the
// object id.
//
#pragma db object abstract
class person
{
  ...

  std::string first_;
  std::string last_;
};

// Abstract employee class. It derives from the person class and
// declares the object id for all the concrete employee types.
//
#pragma db object abstract
class employee: public person
{
  ...

  #pragma db id auto
  unsigned long long id_;
};

// Concrete permanent_employee class. Note that it doesn't define
// any data members of its own.
//
#pragma db object
class permanent_employee: public employee
{
  ...
};

// Concrete temporary_employee class. It adds the employment
// duration in months.
//
#pragma db object
class temporary_employee: public employee
{
  ...

  unsigned long duration_;
};

// Concrete contractor class. It derives from the person class
// (and not employee; an independent contractor is not considered
// an employee). We use the contractor's external email address
// as the object id.
//
#pragma db object
class contractor: public person
{
  ...

  #pragma db id
  std::string email_;
};
  </pre>

  <p>The sample database schema for this hierarchy is shown below.</p>

  <pre class="sql">
CREATE TABLE permanent_employee (
  first TEXT NOT NULL,
  last TEXT NOT NULL,
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT);

CREATE TABLE temporary_employee (
  first TEXT NOT NULL,
  last TEXT NOT NULL,
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
  duration BIGINT UNSIGNED NOT NULL);

CREATE TABLE contractor (
  first TEXT NOT NULL,
  last TEXT NOT NULL,
  email VARCHAR (128) NOT NULL PRIMARY KEY);
  </pre>

  <p>The complete version of the code presented in this section is
     available in the <code>inheritance/reuse</code> example in the
     <code>odb-examples</code> package.</p>

  <h2><a name="8.2">8.2 Polymorphism Inheritance</a></h2>

  <p>There are three general approaches to mapping a polymorphic
     class hierarchy to a relational database. These are
     <em>table-per-hierarchy</em>, <em>table-per-difference</em>,
     and <em>table-per-class</em>. With the table-per-hierarchy
     mapping, all the classes in a hierarchy are stored in a single,
     "wide" table. <code>NULL</code> values are stored in columns
     corresponding to data members of derived classes that are
     not present in any particular instance.</p>

  <p>In the table-per-difference mapping, each class is mapped
     to a separate table. For a derived class, this table contains
     only columns corresponding to the data members added by this
     derived class.</p>

  <p>Finally, in the table-per-class mapping, each class is mapped
     to a separate table. For a derived class, this table contains
     columns corresponding to all the data members, from this derived
     class all the way down to the root of the hierarchy.</p>

  <p>The table-per-difference mapping is generally considered as
     having the best balance of flexibility, performance, and space
     efficiency. It also results in a more canonical relational
     database model compared to the other two approaches. As a
     result, this is the mapping currently implemented in ODB.
     Other mappings may be supported in the future. Note that
     multiple polymorphism inheritance or mixing polymorphism and
     reuse inheritance is not supported.</p>

  <p>A pointer or reference to an ordinary, non-polymorphic object
     has just one type &mdash; the class type of that object. When we
     start working with polymorphic objects, there are two types
     to consider: the <em>static type</em>, or the declaration type
     of a reference or pointer, and the object's actual or <em>dynamic
     type</em>. An example will help illustrate the difference:</p>

  <pre class="cxx">
class person {...};
class employee: public person {...};

person p;
employee e;

person&amp; r1 (p);
person&amp; r2 (e);

unique_ptr&lt;person> p1 (new employee);
  </pre>

  <p>In the above example, the <code>r1</code> reference's both static
     and dynamic types are <code>person</code>.
     In contrast, the <code>r2</code> reference's static type is
     <code>person</code> while its dynamic type (the actual object
     that it refers to) is <code>employee</code>. Similarly,
     <code>p1</code> points to the object of the <code>person</code>
     static type but <code>employee</code> dynamic type.</p>

  <p>In C++, the primary mechanisms for working with polymorphic objects
     are virtual functions. We call a virtual function only knowing the
     object's static type, but the version corresponding to the object's
     dynamic type is automatically executed. This is the essence of
     runtime polymorphism support in C++: we can operate in terms of a base
     class interface but get the derived class' behavior. Similarly, the
     essence of the runtime polymorphism support in ODB is to allow us to
     persist, load, update, and query in terms of the base class interface
     but have the derived class actually stored in the database.</p>

  <p>To declare a persistent class as polymorphic we use the
     <code>db&nbsp;polymorphic</code> pragma. We only need to
     declare the root class of a hierarchy as polymorphic; ODB will
     treat all the derived classes as polymorphic automatically. For
     example:</p>

  <pre class="cxx">
#pragma db object polymorphic
class person
{
  ...

  virtual
  ~person () = 0; // Automatically abstract.

  #pragma db id auto
  unsigned long long id_;

  std::string first_;
  std::string last_;
};

#pragma db object
class employee: public person
{
  ...

  bool temporary_;
};

#pragma db object
class contractor: public person
{

  std::string email_;
};
  </pre>

  <p>A persistent class hierarchy declared polymorphic must also be
     polymorphic in the C++ sense, that is, the root class must
     declare or inherit at least one virtual function. It is
     recommended that the root class also declares a virtual destructor.
     The root class of the polymorphic hierarchy must contain
     the data member designated as object id (a persistent class
     without an object id cannot be polymorphic). Note also that,
     unlike reuse inheritance, abstract polymorphic classes have
     a table in the database, just like non-abstract classes.</p>

  <p>Persistent classes in the same polymorphic hierarchy must use the
     same kind of object pointer (<a href="#3.3">Section 3.3,
     "Object and View Pointers"</a>). If the object pointer
     for the root class is specified as a template or using the
     special raw pointer syntax (<code>*</code>), then the ODB
     compiler will automatically use the same object pointer
     for all the derived classes. For example:</p>

  <pre class="cxx">
#pragma db object polymorphic pointer(std::shared_ptr)
class person
{
  ...
};

#pragma db object // Object pointer is std::shared_ptr&lt;employee>.
class employee: public person
{
  ...
};

#pragma db object // Object pointer is std::shared_ptr&lt;contractor>.
class contractor: public person
{
  ...
};
  </pre>

  <p>Similarly, if we enable or disable session support
     (<a href="#11">Chapter 11, "Session"</a>) for the root class, then
     the ODB compiler will automatically enable or disable it for all
     the derived classes.</p>

  <p>For polymorphic persistent classes, all the database operations can
     be performed on objects with different static and dynamic types.
     Similarly, operations that load persistent objects from the
     database (<code>load()</code>, <code>query()</code>, etc.), can
     return objects with different static and dynamic types. For
     example:</p>

  <pre class="cxx">
unsigned long long id1, id2;

// Persist.
//
{
  shared_ptr&lt;person> p1 (new employee (...));
  shared_ptr&lt;person> p2 (new contractor (...));

  transaction t (db.begin ());
  id1 = db.persist (p1); // Stores employee.
  id2 = db.persist (p2); // Stores contractor.
  t.commit ();
}

// Load.
//
{
  shared_ptr&lt;person> p;

  transaction t (db.begin ());
  p = db.load&lt;person> (id1); // Loads employee.
  p = db.load&lt;person> (id2); // Loads contractor.
  t.commit ();
}

// Query.
//
{
  using query = odb::query&lt;person>;
  using result = odb::result&lt;person>;

  transaction t (db.begin ());

  result r (db.query&lt;person> (query::last == "Doe"));

  for (result::iterator i (r.begin ()); i != r.end (); ++i)
  {
    person&amp; p (*i); // Can be employee or contractor.
  }

  t.commit ();
}

// Update.
//
{
  shared_ptr&lt;person> p;
  shared_ptr&lt;employee> e;

  transaction t (db.begin ());

  e = db.load&lt;employee> (id1);
  e->temporary (false);
  p = e;
  db.update (p); // Updates employee.

  t.commit ();
}

// Erase.
//
{
  shared_ptr&lt;person> p;

  transaction t (db.begin ());
  p = db.load&lt;person> (id1); // Loads employee.
  db.erase (p);              // Erases employee.
  db.erase&lt;person> (id2);    // Erases contractor.
  t.commit ();
}
  </pre>


  <p>The table-per-difference mapping, as supported by ODB, requires
     two extra columns, in addition to those corresponding to the
     data members. The first, called <em>discriminator</em>, is added
     to the table corresponding to the root class of the hierarchy.
     This column is used to determine the dynamic type of each
     object. The second column is added to tables corresponding
     to the derived classes and contains the object id. This
     column is used to form a foreign key constraint referencing
     the root class table.</p>

  <p>When querying the database for polymorphic objects, it is
     possible to obtain the discriminator value without
     instantiating the object. For example:</p>

  <pre class="cxx">
using query = odb::query&lt;person>;
using result = odb::result&lt;person>;

transaction t (db.begin ());

result r (db.query&lt;person> (query::last == "Doe"));

for (result::iterator i (r.begin ()); i != r.end (); ++i)
{
  std::string d (i.discriminator ());
  ...
}

t.commit ();
  </pre>

  <p>In the current implementation, ODB has limited support for
     customizing names, types, and values of the extra columns.
     Currently, the discriminator column is always called
     <code>typeid</code> and contains a namespace-qualified class
     name (for example, <code>"employee"</code> or
     <code>"hr::employee"</code>). The id column in the derived
     class table has the same name as the object id column in
     the root class table. Future versions of ODB will add support
     for customizing these extra columns.</p>

  <p>The sample database schema for the above polymorphic hierarchy
     is shown below.</p>

  <pre class="sql">
CREATE TABLE person (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
  typeid VARCHAR(128) NOT NULL,
  first TEXT NOT NULL,
  last TEXT NOT NULL);

CREATE TABLE employee (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY,
  temporary TINYINT(1) NOT NULL,

  CONSTRAINT employee_id_fk
    FOREIGN KEY (id)
    REFERENCES person (id)
    ON DELETE CASCADE);

CREATE TABLE contractor (
  id BIGINT UNSIGNED NOT NULL PRIMARY KEY,
  email TEXT NOT NULL,

  CONSTRAINT contractor_id_fk
    FOREIGN KEY (id)
    REFERENCES person (id)
    ON DELETE CASCADE);
  </pre>

  <p>The complete version of the code presented in this section is
     available in the <code>inheritance/polymorphism</code> example
     in the <code>odb-examples</code> package.</p>

  <h3><a name="8.2.1">8.2.1 Performance and Limitations</a></h3>

  <p>A database operation on a non-polymorphic object normally translates
     to a single database statement execution (objects with containers
     and eager object pointers can be the exception). Because polymorphic
     objects have their data members
     stored in multiple tables, some database operations on such objects
     may result in multiple database statements being executed while others
     may require more complex statements. There is also some functionality
     that is not available to polymorphic objects.</p>

  <p>The first part of this section discusses the performance implications
     to keep in mind when designing and working with polymorphic hierarchies.
     The second part talks about limitations of polymorphic objects.</p>

  <p>The most important aspect of a polymorphic hierarchy that
     affects database performance is its depth. The distance between
     the root of the hierarchy and the derived class translates
     directly to the number of database statements that will have to
     be executed in order to persist, update, or erase this derived class.
     It also translates directly to the number of SQL <code>JOIN</code>
     clauses that will be needed to load or query the database for this
     derived class. As a result, to achieve best performance, we should
     try to keep our polymorphic hierarchies as flat as possible.</p>

  <p>When loading an object or querying the database for objects,
     ODB will need to execute two statements if this object's static
     and dynamic types are different but only one statement if
     they are the same. This example will help illustrate the
     difference:</p>

  <pre class="cxx">
unsigned long long id;

{
  employee e (...);

  transaction t (db.begin ());
  id = db.persist (e);
  t.commit ();
}

{
  shared_ptr&lt;person> p;

  transaction t (db.begin ());
  p = db.load&lt;person> (id);   // Requires two statement.
  p = db.load&lt;employee> (id); // Requires only one statement.
  t.commit ();
}
  </pre>

  <p>As a result, we should try to load and query using the most
     derived class possible.</p>

  <p>Finally, for polymorphic objects, erasing via the object instance
     is faster than erasing via its object id. In the former case the
     object's dynamic type can be determined locally in the application
     while in the latter case an extra statement has to be executed to
     achieve the same result. For example:</p>

  <pre class="cxx">
shared_ptr&lt;person> p = ...;

transaction t (db.begin ());
db.erase&lt;person> (p.id ()); // Slower (executes extra statement).
db.erase (p);               // Faster.
t.commit ();
  </pre>

  <p>Polymorphic objects can use all the mechanisms that are available
     to ordinary objects. These include containers (<a href="#5">Chapter 5,
     "Containers"</a>), object relationships, including to polymorphic
     objects (<a href="#6">Chapter 6, "Relationships"</a>), views
     (<a href="#10">Chapter 10, "Views"</a>), session (<a href="#11">Chapter
     11, "Session"</a>), and optimistic concurrency (<a href="#12">Chapter
     12, "Optimistic Concurrency"</a>). There are, however, a few
     limitations, mainly due to the underlying use of SQL to access the
     data.</p>

  <p>When a polymorphic object is "joined" in a view, and the join
     condition (either in the form of an object pointer or a custom
     condition) comes from the object itself (as opposed to one of
     the objects joined previously), then this condition must only
     use data members from the derived class. For example, consider
     the following polymorphic object hierarchy and a view:</p>


  <pre class="cxx">
#pragma db object polymorphic
class employee
{
  ...
};

#pragma db object
class permanent_employee: public employee
{
  ...
};

#pragma db object
class temporary_employee: public employee
{
  ...

  shared_ptr&lt;permanent_employee> manager_;
};

#pragma db object
class contractor: public temporary_employee
{
  shared_ptr&lt;permanent_employee> manager_;
};

#pragma db view object(permanent_employee) \
                object(contractor: contractor::manager_)
struct contractor_manager
{
  ...
};
  </pre>

  <p>This view will not function correctly because the join condition
     (<code>manager_</code>) comes from the base class
     (<code>temporary_employee</code>) instead of the derived
     (<code>contractor</code>). The reason for this limitation is the
     <code>JOIN</code> clause order in the underlying SQL <code>SELECT</code>
     statement. In the view presented above, the table corresponding
     to the base class (<code>temporary_employee</code>) will have to
     be joined first which will result in this view matching both
     the <code>temporary_employee</code> and <code>contractor</code>
     objects instead of just <code>contractor</code>. It is usually
     possible to resolve this issue by reordering the objects in the
     view. Our example, for instance, can be fixed by swapping the
     two objects:</p>

  <pre class="cxx">
#pragma db view object(contractor) \
                object(permanent_employee: contractor::manager_)
struct contractor_manager
{
  ...
};
  </pre>

  <p>The <code>erase_query()</code> database function (<a href="#3.11">Section
     3.11, "Deleting Persistent Objects"</a>) also has limited functionality
     when used on polymorphic objects. Because many database implementations
     do not support <code>JOIN</code> clauses in the SQL <code>DELETE</code>
     statement, only data members from the derived class being erased can
     be used in the query condition. For example:</p>

  <pre class="cxx">
using query = odb::query&lt;employee>;

transaction t (db.begin ());
db.erase_query&lt;employee> (query::permanent);     // Ok.
db.erase_query&lt;employee> (query::last == "Doe"); // Error.
t.commit ();
  </pre>

  <h2><a name="8.3">8.3 Mixed Inheritance</a></h2>

  <p>It is possible to mix the reuse and polymorphism inheritance
     styles in the same hierarchy. In this case, the reuse inheritance
     must be used for the "bottom" (base) part of the hierarchy while
     the polymorphism inheritance &mdash; for the "top" (derived) part.
     For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
};

#pragma db object polymorphic
class employee: public person // Reuse inheritance.
{
  ...
};

#pragma db object
class temporary_employee: public employee // Polymorphism inheritance.
{
  ...
};

#pragma db object
class permanent_employee: public employee // Polymorphism inheritance.
{
  ...
};
  </pre>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="9">9 Sections</a></h1>

  <p>ODB sections are an optimization mechanism that allows us to
     partition data members of a persistent class into groups that
     can be separately loaded and/or updated. This can be useful,
     for example, if an object contains expensive to load or update
     data members (such as <code>BLOB</code>s or containers) and
     that are accessed or modified infrequently. For example:</p>

  <pre class="cxx">
#include &lt;odb/section.hxx>

#pragma db object
class person
{
  ...

  #pragma db load(lazy) update(manual)
  odb::section keys_;

  #pragma db section(keys_) type("BLOB")
  char public_key_[1024];

  #pragma db section(keys_) type("BLOB")
  char private_key_[1024];
};

transaction t (db.begin ());

unique_ptr&lt;person> p (db.load&lt;person> (...)); // Keys are not loaded.

if (need_keys)
{
  db.load (*p, p->keys_); // Load keys.
  ...
}

db.update (*p); // Keys are not updated.

if (update_keys)
{
  ...
  db.update (*p, p->keys_); // Update keys.
}

t.commit ();
  </pre>

  <p>A complete example that shows how to use sections is available in
     the <code>section</code> directory in the <code>odb-examples</code>
     package.</p>

  <p>Why do we need to group data members into sections? Why can't
     each data member be loaded and updated independently if and
     when necessary? The reason for this requirement is that loading
     or updating a group of data members with a single database
     statement is significantly more efficient than loading or updating
     each data member with a separate statement. Because ODB
     prepares and caches statements used to load and update
     persistent objects, generating a custom statement for
     a specific set of data members that need to be loaded or
     updated together is not a viable approach either. To resolve
     this, ODB allows us to group data members that are
     often updated and/or loaded together into sections. To
     achieve the best performance, we should aim to find a balance
     between having too many sections with too few data
     members and too few sections with too many data
     members. We can use the access and modification patterns
     of our application as a base for this decision.</p>

  <p>To add a new section to a persistent class we declare a new
     data member of the <code>odb::section</code> type. At this
     point we also need to specify the loading and updating behavior
     of this section with the <code>db&nbsp;load</code> and
     <code>db&nbsp;update</code> pragmas, respectively.</p>

  <p>The loading behavior of a section can be either <code>eager</code>
     or <code>lazy</code>. An eager-loaded section is always loaded as
     part of the object load. A lazy-loaded section is not loaded
     as part of the object load and has to be explicitly loaded with
     the <code>database::load()</code> function (discussed below) if
     and when necessary.</p>

  <p>The updating behavior of a section can be <code>always</code>,
     <code>change</code>, or <code>manual</code>. An always-updated
     section is always updated as part of the object update,
     provided it has been loaded. A change-updated section
     is only updated as part of the object update if it has been loaded
     and marked as changed. A manually-updated section is never updated
     as part of the object update and has to be explicitly updated with
     the <code>database::update()</code> function (discussed below) if
     and when necessary.</p>

  <p>If no loading behavior is specified explicitly, then an eager-loaded
     section is assumed. Similarly, if no updating behavior is specified,
     then an always-updated section is assumed. An eager-loaded, always-updated
     section is pointless and therefore illegal. Only persistent classes
     with an object id can have sections.</p>

  <p>To specify that a data member belongs to a section we use the
     <code>db&nbsp;section</code> pragma with the section's member
     name as its single argument. Except for special data members
     such as the object id and optimistic concurrency version, any
     direct, non-transient member of a persistent class can belong
     to a section, including composite values, containers, and
     pointers to objects. For example:</p>

  <pre class="cxx">
#pragma db value
class text
{
  std::string data;
  std::string lang;
};

#pragma db object
class person
{
  ...

  #pragma db load(lazy)
  odb::section extras_;

  #pragma db section(extras_)
  text bio_;

  #pragma db section(extras_)
  std::vector&lt;std::string> nicknames_;

  #pragma db section(extras_)
  std::shared_ptr&lt;person> emergency_contact_;
};
  </pre>

  <p>An empty section is pointless and therefore illegal, except
     in abstract or polymorphic classes where data members can be
     added to a section by derived classes (see <a href="#9.1">Section
     9.1, "Sections and Inheritance"</a>).</p>

  <p>The <code>odb::section</code> class is defined in the
     <code>&lt;odb/section.hxx></code> header file and has the
     following interface:</p>

  <pre class="cxx">
namespace odb
{
  class section
  {
  public:
    // Load state.
    //
    bool
    loaded () const;

    void
    unload ();

    void
    load ();

    // Change state.
    //
    bool
    changed () const;

    void
    change ();

    // User data.
    //
    unsigned char
    user_data () const;

    void
    user_data (unsigned char);
  };
}
  </pre>

  <p>The <code>loaded()</code> accessor can be used to determine whether a
     section is already loaded. The <code>unload()</code> modifier marks a
     loaded section as not loaded. This, for example, can be useful if you
     don't want the section to be reloaded during the object reload. The
     <code>load()</code> modifier marks an unloaded section as loaded
     without actually loading any of its data members. This, for example,
     can be useful if you don't want to load the old state before overwriting
     it with <code>update()</code>.</p>

  <p>The <code>changed()</code> accessor can be used to query the
     section's change state. The <code>change()</code> modifier
     marks the section as changed. It is valid to call this modifier
     for an unloaded (or transient) section, however, the state will
     be reset back to unchanged once the section (or object) is loaded.
     The change state is only relevant to sections with change-updated
     behavior and is ignored for all other sections.</p>

  <p>The size of the section class is one byte with four bits available
     to store a custom state via the <code>user_data()</code> accessor
     and modifier.</p>

  <p>The <code>odb::database</code> class provides special
     versions of the <code>load()</code> and <code>update()</code>
     functions that allow us to load and update sections of a
     persistent class. Their signatures are as follows:</p>

  <pre class="cxx">
  template &lt;typename T>
  void
  load (T&amp; object, section&amp;);

  template &lt;typename T>
  void
  update (const T&amp; object, const section&amp;);
  </pre>

  <p>Before calling the section <code>load()</code> function, the
     object itself must already be loaded. If the section is already
     loaded, then the call to <code>load()</code> will reload its
     data members. It is illegal to explicitly load an eager-loaded
     section.</p>

  <p>Before calling the section <code>update()</code> function, the
     section (and therefore the object) must be in the loaded state.
     If the section is not loaded, the <code>odb::section_not_loaded</code>
     exception is thrown. The section <code>update()</code> function
     does not check but does clear the section's change state. In
     other words, section <code>update()</code> will always update
     section data members in the database and clear the change flag.
     Note also that any section, that is, always-, change-, or
     manually-updated, can be explicitly updated with this function.</p>

  <p>Both section <code>load()</code> and <code>update()</code>, just
     like the rest of the database operations, must be performed within
     a transaction. Notice also that both <code>load()</code> and
     <code>update()</code> expect a reference to the section as
     their second argument. This reference must refer to the data
     member in the object passed as the first argument. If instead
     it refers to some other instance of the <code>section</code>
     class, for example, a local copy or a temporary, then the
     <code>odb::section_not_in_object</code> exception is thrown.
     For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
public:
  ...

  odb::section
  keys () const {return keys_;}

private:
  odb::section keys_;

  ...
};

unique_ptr&lt;person> p (db.load&lt;person> (...));

section s (p->keys ());
db.load (*p, s);            // Throw section_not_in_object, copy.

db.update (*p, p->keys ()); // Throw section_not_in_object, copy.
  </pre>

  <p>At first glance it may seem more appropriate to make the
     <code>section</code> class non-copyable in order to prevent
     such errors from happening. However, it is perfectly reasonable
     to expect to be able to copy (or assign) sections as part of
     the object copying (or assignment). As a result, sections are
     left copyable and copy-assignable, however, this functionality
     should not be used in accessors or modifiers. Instead, section
     accessors and modifiers should always be by-reference. Here is
     how we can fix our previous example:</p>

<pre class="cxx">
#pragma db object
class person
{
public:
  ...

  const odb::section&amp;
  keys () const {return keys_;}

  odb::section&amp;
  keys () {return keys_;}

private:
  odb::section keys_;

  ...
};

unique_ptr&lt;person> p (db.load&lt;person> (...));

section&amp; s (p->keys ());
db.load (*p, s);            // Ok, reference.

db.update (*p, p->keys ()); // Ok, reference.
  </pre>

  <p>Several other database operations affect sections. The state of
     a section in a transient object is undefined. That is, before
     the call to object <code>persist()</code> or <code>load()</code>
     functions, or after the call to object <code>erase()</code>
     function, the values returned by the <code>section::loaded()</code> and
     <code>section::changed()</code> accessors are undefined.</p>

  <p>After the call to <code>persist()</code>, all sections, including
     eager-loaded ones, are marked as loaded and unchanged. If instead we
     are loading an object with the <code>load()</code> call or as
     a result of a query, then eager-loaded sections are loaded
     and marked as loaded and unchanged while lazy-loaded ones are marked
     as unloaded. If a lazy-loaded section is later loaded with the
     section <code>load()</code> call, then it is marked as loaded and
     unchanged.</p>

  <p>When we update an object with the <code>update()</code> call,
     manually-updated sections are ignored while always-updated
     sections are updated if they are loaded. Change-updated
     sections are only updated if they are both loaded and marked
     as changed. After the update, such sections are reset to the
     unchanged state. When we reload an object with the
     <code>reload()</code> call, sections that were loaded are
     automatically reloaded and reset to the unchanged state.</p>

  <p>To further illustrate the state transitions of a section,
     consider this example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db load(lazy) update(change)
  odb::section keys_;

  ...
};

transaction t (db.begin ());

person p ("John", "Doe"); // Section state is undefined (transient).

db.persist (p);           // Section state: loaded, unchanged.

unique_ptr&lt;person> l (
  db.load&lt;person> (...)); // Section state: unloaded, unchanged.

db.update (*l);           // Section not updated since not loaded.
db.update (p);            // Section not updated since not changed.

p.keys_.change ();        // Section state: loaded, changed.
db.update (p);            // Section updated, state: loaded, unchanged.

db.update (*l, l->keys_); // Throw section_not_loaded.
db.update (p, p.keys_);   // Section updated even though not changed.

db.reload (*l);           // Section not reloaded since not loaded.
db.reload (p);            // Section reloaded, state: loaded, unchanged.

db.load (*l, l->keys_);   // Section loaded, state: loaded, unchanged.
db.load (p, p.keys_);     // Section reloaded, state: loaded, unchanged.

db.erase (p);             // Section state is undefined (transient).

t.commit ();
  </pre>

  <p>When using change-updated behavior, it is our responsibility to
     mark the section as changed when any of the data members belonging
     to this section is modified. A natural place to mark the section
     as changed is the modifiers for section data members, for example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  using key_type = std::array&lt;char, 1024>;

  const key_type&amp;
  public_key () const {return public_key_;}

  void
  public_key (const key_type&amp; k)
  {
    public_key_ = k;
    keys_.change ();
  }

  const key_type&amp;
  private_key () const {return private_key_;}

  void
  private_key (const key_type&amp; k)
  {
    private_key_ = k;
    keys_.change ();
  }

private:
  #pragma db load(lazy) update(change)
  odb::section keys_;

  #pragma db section(keys_) type("BLOB")
  key_type public_key_;

  #pragma db section(keys_) type("BLOB")
  key_type private_key_;

  ...
};
  </pre>

  <p>One interesting aspect of change-updated sections is what happens
     when a transaction that performed an object or section update is
     later rolled back. In this case, while the change state of a
     section has been reset (after update), actual changes were not
     committed to the database. Change-updated sections handle this
     case by automatically registering a rollback callback and then,
     if it is called, restoring the original change state. The
     following code illustrates this semantics (continuing with
     the previous example):</p>

  <pre class="cxx">
unique_ptr&lt;person> p;

try
{
  transaction t (db.begin ());
  p = db.load&lt;person> (...);
  db.load (*p, p->keys_);

  p->private_key (new_key); // The section is marked changed.
  db.update (*p);           // The section is reset to unchanged.

  throw failed ();          // Triggers rollback.
  t.commit ();
}
catch (const failed&amp;)
{
  // The section is restored back to changed.
}
  </pre>


  <h2><a name="9.1">9.1 Sections and Inheritance</a></h2>

  <p>With both reuse and polymorphism inheritance (<a href="#8">Chapter 8,
     "Inheritance"</a>) it is possible to add new sections to derived
     classes. It is also possible to add data members from derived
     classes to sections declared in the base. For example:</p>

  <pre class="cxx">
#pragma db object polymorphic
class person
{
  ...

  virtual void
  print ();

  #pragma db load(lazy)
  odb::section print_;

  #pragma db section(print_)
  std::string bio_;
};

#pragma db object
class employee: public person
{
  ...

  virtual void
  print ();

  #pragma db section(print_)
  std::vector&lt;std::string> employment_history_;
};

transaction t (db.begin ());

unique_ptr&lt;person> p (db.load&lt;person> (...)); // Person or employee.
db.load (*p, p->print_); // Load data members needed for print.
p->print ();

t.commit ();
  </pre>

  <p>When data members of a section are spread over several classes in a
     reuse inheritance hierarchy, both section load and update are
     performed with a single database statement. In contrast, with
     polymorphism inheritance, section load is performed with a
     single statement while update requires a separate statement
     for each class that adds to the section.</p>

  <p>Note also that in polymorphism inheritance the section-to-object
     association is static. Or, in other words, you can load a section
     via an object only if its static type actually contains this
     section. The following example will help illustrate this
     point further:</p>

  <pre class="cxx">
#pragma db object polymorphic
class person
{
  ...
};

#pragma db object
class employee: public person
{
  ...

  #pragma db load(lazy)
  odb::section extras_;

  ...
};

#pragma db object
class manager: public employee
{
  ...
};

unique_ptr&lt;manager> m (db.load&lt;manager> (...));

person&amp; p (*m);
employee&amp; e (*m);
section&amp; s (m->extras_);

db.load (p, s); // Error: extras_ is not in person.
db.load (e, s); // Ok: extras_ is in employee.
  </pre>

  <h2><a name="9.2">9.2 Sections and Optimistic Concurrency</a></h2>

  <p>When sections are used in a class with the optimistic concurrency
     model (<a href="#12">Chapter 12, "Optimistic Concurrency"</a>),
     both section update and load operations compare the object version
     to that in the database and throw the <code>odb::object_changed</code>
     exception if they do not match. In addition, the section update
     operation increments the version to indicate that the object state
     has changed. For example:</p>

  <pre class="cxx">
#pragma db object optimistic
class person
{
  ...

  #pragma db version
  unsigned long long version_;

  #pragma db load(lazy)
  odb::section extras_;

  #pragma db section(extras_)
  std::string bio_;
};

unique_ptr&lt;person> p;

{
  transaction t (db.begin ());
  p = db.load&lt;person> (...);
  t.commit ();
}

{
  transaction t (db.begin ());

  try
  {
    db.load (*p, p->extras_); // Throws if object state has changed.
  }
  catch (const object_changed&amp;)
  {
    db.reload (*p);
    db.load (*p, p->extras_); // Cannot fail.
  }

  t.commit ();
}
  </pre>

  <p>Note also that if an object update triggers one or more
     section updates, then each such update will increment the
     object version. As a result, an update of an object that
     contains sections may result in a version increment by
     more than one.</p>

  <p>When sections are used together with optimistic concurrency and
     inheritance, an extra step may be required to enable this
     functionality. If you plan to add new sections to derived
     classes, then the root class of the hierarchy
     (the one that declares the version data member) must be
     declared as sectionable with the <code>db&nbsp;sectionable</code>
     pragma. For example:</p>

  <pre class="cxx">
#pragma db object polymorphic sectionable
class person
{
  ...

  #pragma db version
  unsigned long long version_;
};

#pragma db object
class employee: public person
{
  ...

  #pragma db load(lazy)
  odb::section extras_;

  #pragma db section(extras_)
  std::vector&lt;std::string> employment_history_;
};
  </pre>

  <p>This requirement has to do with the need to generate extra
     version increment code in the root class that will be used
     by sections added in the derived classes. If you forget to
     declare the root class as sectionable and later add a
     section to one of the derived classes, the ODB compiler
     will issue diagnostics.</p>

  <h2><a name="9.3">9.3 Sections and Lazy Pointers</a></h2>

  <p>If a lazy pointer (<a href="#6.4">Section 6.4, "Lazy Pointers"</a>)
     belongs to a lazy-loaded section, then we end up with two levels of
     lazy loading. Specifically, when the section is loaded, the lazy
     pointer is initialized with the object id but the object itself
     is not loaded. For example:</p>

  <pre class="cxx">
#pragma db object
class employee
{
  ...

  #pragma db load(lazy)
  odb::section extras_;

  #pragma db section(extras_)
  odb::lazy_shared_ptr&lt;employer> employer_;
};

transaction t (db.begin ());

unique_ptr&lt;employee> e (db.load&lt;employee> (...)); // employer_ is NULL.

db.load (*e, e->extras_); // employer_ contains valid employer id.

e->employer_.load (); // employer_ points to employer object.

t.commit ();
  </pre>

  <h2><a name="9.4">9.4 Sections and Change-Tracking Containers</a></h2>

  <p>If a change-tracking container (<a href="#5.4">Section 5.4,
     "Change-Tracking Containers"</a>) belongs to a change-updated
     section, then prior to an object update ODB will check if the
     container has been changed and if so, automatically mark the
     section as changed. For example:</p>

<pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db load(lazy) update(change)
  odb::section extras_;

  #pragma db section(extras_)
  odb::vector&lt;std::string> nicknames_;
};

transaction t (db.begin ());

unique_ptr&lt;person> p (db.load&lt;person> (...));
db.load (*p, p->extras_);

p->nicknames_.push_back ("JD");

db.update (*p); // Section is automatically updated even
                // though it was not marked as changed.
t.commit ();
  </pre>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="10">10 Views</a></h1>

  <p>An ODB view is a C++ <code>class</code> or <code>struct</code> type
     that embodies a light-weight, read-only projection of one or more
     persistent objects or database tables or the result of a native SQL
     query execution.</p>

  <p>Some of the common applications of views include loading a subset
     of data members from objects or columns from database tables, executing
     and handling results of arbitrary SQL queries, including aggregate
     queries and stored procedure calls, as well as joining multiple
     objects and/or database tables using object relationships or custom
     join conditions.</p>

  <p>Many relational databases also define the concept of views. Note,
     however, that ODB views are not mapped to database views. Rather,
     by default, an ODB view is mapped to an SQL <code>SELECT</code>
     query. However, if desired, it is easy to create an ODB view
     that is based on a database view.</p>

  <p>Usually, views are defined in terms of other persistent entities,
     such as persistent objects, database tables, sequences, etc.
     Therefore, before we can examine our first view, we need to
     define a few persistent objects and a database table. We will
     use this model in examples throughout this chapter. Here we
     assume that you are familiar with ODB object relationship
     support (<a href="#6">Chapter 6, "Relationships"</a>).</p>

  <pre class="cxx">
#pragma db object
class country
{
  ...

  #pragma db id
  std::string code_; // ISO 2-letter country code.

  std::string name_;
};

#pragma db object
class employer
{
  ...

  #pragma db id
  unsigned long long id_;

  std::string name_;
};

#pragma db object
class employee
{
  ...

  #pragma db id
  unsigned long long id_;

  std::string first_;
  std::string last_;

  unsigned short age_;

  shared_ptr&lt;country> residence_;
  shared_ptr&lt;country> nationality_;

  shared_ptr&lt;employer> employed_by_;
};
  </pre>

  <p>Besides these objects, we also have the legacy
     <code>employee_extra</code> table that is not mapped to any persistent
     class. It has the following definition:</p>

  <pre class="sql">
CREATE TABLE employee_extra(
  employee_id INTEGER NOT NULL,
  vacation_days INTEGER NOT NULL,
  previous_employer_id INTEGER)
  </pre>

  <p>The above persistent objects and database table as well as many of
     the views shown in this chapter are based on the
     <code>view</code> example which can be found in the
     <code>odb-examples</code> package of the ODB distribution.</p>

  <p>To declare a view we use the <code>db&nbsp;view</code> pragma,
     for example:</p>

  <pre class="cxx">
#pragma db view object(employee)
struct employee_name
{
  std::string first;
  std::string last;
};
  </pre>

  <p>The above example shows one of the simplest views that we can create.
     It has a single associated object (<code>employee</code>) and its
     purpose is to extract the employee's first and last names without
     loading any other data, such as the referenced <code>country</code>
     and <code>employer</code> objects.</p>

  <p>Views use the same query facility (<a href="#4">Chapter 4, "Querying
     the Database"</a>) as persistent objects. Because support for queries
     is optional and views cannot be used without this support, you need
     to compile any header that defines a view with the
     <code>--generate-query</code> ODB compiler option.</p>

  <p>To query the database for a view we use the
     <code>database::query()</code>, <code>database::query_one()</code>, or
     <code>database::query_value()</code> functions in exactly the same way
     as we would use them to query the database for an object. For example,
     the following code fragment shows how we can find the names of all the
     employees that are younger than 31:</p>

  <pre class="cxx">
using query = odb::query&lt;employee_name>;
using result = odb::result&lt;employee_name>;

transaction t (db.begin ());

result r (db.query&lt;employee_name> (query::age &lt; 31));

for (result::iterator i (r.begin ()); i != r.end (); ++i)
{
  const employee_name&amp; en (*i);
  cout &lt;&lt; en.first &lt;&lt; " " &lt;&lt; en.last &lt;&lt; endl;
}

t.commit ();
  </pre>

  <p>A view can be defined as a projection of one or more objects, one
     or more tables, a combination of objects and tables, or it can be
     the result of a custom SQL query. The following sections discuss each
     of these kinds of view in more detail.</p>

  <h2><a name="10.1">10.1 Object Views</a></h2>

  <p>To associate one or more objects with a view we use the
     <code>db&nbsp;object</code> pragma (<a href="#14.2.1">Section
     14.2.1, "<code>object</code>"</a>). We have already seen
     a simple, single-object view in the introduction to this chapter.
     To associate the second and subsequent objects we repeat the
     <code>db&nbsp;object</code> pragma for each additional object,
     for example:</p>

  <pre class="cxx">
#pragma db view object(employee) object(employer)
struct employee_employer
{
  std::string first;
  std::string last;
  std::string name;
};
  </pre>

  <p>The complete syntax of the <code>db&nbsp;object</code> pragma is
     shown below:</p>

  <p><code><b>object(</b><i>name</i>
                     [<b>=</b> <i>alias</i>]
                     [<i>join-type</i>]
                     [<b>:</b> <i>join-condition</i>]<b>)</b></code></p>

  <p>The <i>name</i> part is a potentially qualified persistent class
     name that has been defined previously. The optional <i>alias</i>
     part gives this object an alias. If provided, the alias is used
     in several contexts instead of the object's unqualified name. We
     will discuss aliases further as we cover each of these contexts
     below. The optional <i>join-type</i> part specifies the way this
     object is associated. It can be <code>left</code>, <code>right</code>,
     <code>full</code>, <code>inner</code>, and <code>cross</code>
     with <code>left</code> being the default.
     Finally, the optional <i>join-condition</i> part provides the
     criteria which should be used to associate this object with any
     of the previously associated objects or, as we will see in
     <a href="#10.4">Section 10.4, "Mixed Views"</a>, tables. Note that
     while the first associated object can have an alias, it cannot
     have a join type or condition.</p>

  <p>For each subsequent associated object the ODB compiler needs
     a join condition and there are several ways to specify
     it. The easiest way is to omit it altogether and let the ODB
     compiler try to come up with a join condition automatically.
     To do this the ODB compiler will examine each previously
     associated object for object relationships
     (<a href="#6">Chapter 6, "Relationships"</a>) that
     may exist between these objects and the object being associated.
     If such a relationship exists and is unambiguous, that is
     there is only one such relationship, then the ODB compiler
     will automatically use it to come up with the join condition for
     this object. This is exactly what happens in the previous
     example: there is a single relationship
     (<code>employee::employed_by</code>) between the
     <code>employee</code> and <code>employer</code> objects.</p>

  <p>On the other hand, consider this view:</p>

  <pre class="cxx">
#pragma db view object(employee) object(country)
struct employee_residence
{
  std::string first;
  std::string last;
  std::string name;
};
  </pre>

  <p>While there is a relationship between <code>country</code> and
     <code>employee</code>, it is ambiguous. It can be
     <code>employee::residence_</code> (which is what we want) or
     it can be <code>employee::nationality_</code> (which we don't
     want). As result, when compiling the above view, the ODB
     compiler will issue an error indicating an ambiguous object
     relationship. To resolve this ambiguity, we can explicitly
     specify the object relationship that should be used to create
     the join condition as the name of the corresponding data member.
     Here is how we can fix the <code>employee_residence</code>
     view:</p>

  <pre class="cxx">
#pragma db view object(employee) object(country: employee::residence_)
struct employee_residence
{
  std::string first;
  std::string last;
  std::string name;
};
  </pre>

  <p>It is possible to associate the same object with a single view
     more than once using different join conditions. However, in
     this case, we have to use aliases to assign different names
     for each association. For example:</p>

  <pre class="cxx">
#pragma db view object(employee) \
  object(country = res_country: employee::residence_) \
  object(country = nat_country: employee::nationality_)
struct employee_country
{
  ...
};
  </pre>

  <p>Note that correctly defining data members in this view requires
     the use of a mechanism that we haven't yet covered. We will
     see how to do this shortly.</p>

  <p>If we assign an alias to an object and refer to a data member of
     this object in one of the join conditions, we have to use the
     unqualified alias name instead of the potentially qualified
     object name. For example:</p>

  <pre class="cxx">
#pragma db view object(employee = ee) object(country: ee::residence_)
struct employee_residence
{
  ...
};
  </pre>

  <p>The last way to specify a join condition is to provide a custom
     query expression. This method is primarily useful if you would
     like to associate an object using a condition that does not
     involve an object relationship. Consider, for example, a
     modified <code>employee</code> object from the beginning of
     the chapter with an added country of birth member. For one
     reason or another we have decided not to use a relationship to
     the <code>country</code> object, as we have done with
     residence and nationality.</p>

  <pre class="cxx">
#pragma db object
class employee
{
  ...

  std::string birth_place_; // Country name.
};
  </pre>

  <p>If we now want to create a view that returns the birth country code
     for an employee, then we have to use a custom join condition when
     associating the <code>country</code> object. For example:</p>

  <pre class="cxx">
#pragma db view object(employee) \
  object(country: employee::birth_place_ == country::name_)
struct employee_birth_code
{
  std::string first;
  std::string last;
  std::string code;
};
  </pre>

  <p>The syntax of the query expression in custom join conditions
     is the same as in the query facility used to query the database
     for objects (<a href="#4">Chapter 4, "Querying the Database"</a>)
     except that for query members, instead of using
     <code>odb::query&lt;object>::member</code> names, we refer directly
     to object members.</p>

  <p>Looking at the views we have defined so far, you may be wondering
     how the ODB compiler knows which view data members correspond to which
     object data members. While the names are similar, they are not exactly
     the same, for example <code>employee_name::first</code> and
     <code>employee::first_</code>.</p>

  <p>As with join conditions, when it comes to associating data members,
     the ODB compiler tries to do this automatically. It first searches
     all the associated objects for an exact name match. If no match is
     found, then the ODB compiler compares the so-called public names.
     A public name of a member is obtained by removing the common member
     name decorations, such as leading and trailing underscores, the
     <code>m_</code> prefix, etc. In both of these searches the ODB
     compiler also makes sure that the types of the two members are the
     same or compatible.</p>

  <p>If one of the above searches returned a match and it is unambiguous, that
     is there is only one match, then the ODB compiler will automatically
     associate the two members. On the other hand, if no match is found
     or the match is ambiguous, the ODB compiler will issue an error.
     To associate two differently-named members or to resolve an ambiguity,
     we can explicitly specify the member association using the
     <code>db&nbsp;column</code> pragma (<a href="#14.4.9">Section 14.4.9,
     "<code>column</code>"</a>). For example:</p>

  <pre class="cxx">
#pragma db view object(employee) object(employer)
struct employee_employer
{
  std::string first;
  std::string last;

  #pragma db column(employer::name_)
  std::string employer_name;
};
  </pre>

  <p>If an object data member specifies the SQL type with
     the <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section
     14.4.3, "<code>type</code>"</a>), then this type is also used for
     the associated view data members.</p>

  <p>Note also that similar to join conditions, if we assign an alias to
     an object and refer to a data member of this object in one of the
     <code>db&nbsp;column</code> pragmas, then we have to use the
     unqualified alias name instead of the potentially qualified
     object name. For example:</p>

  <pre class="cxx">
#pragma db view object(employee) \
  object(country = res_country: employee::residence_) \
  object(country = nat_country: employee::nationality_)
struct employee_country
{
  std::string first;
  std::string last;

  #pragma db column(res_country::name_)
  std::string res_country_name;

  #pragma db column(nat_country::name_)
  std::string nat_country_name;
};
  </pre>

  <p>Besides specifying just the object member, we can also specify a
     <em>+-expression</em> in the <code>db&nbsp;column</code> pragma. A
     +-expression consists of string literals and object
     member references connected using the <code>+</code> operator.
     It is primarily useful for defining aggregate views based on
     SQL aggregate functions, for example:</p>

  <pre class="cxx">
#pragma db view object(employee)
struct employee_count
{
  #pragma db column("count(" + employee::id_ + ")")
  std::size_t count;
};
  </pre>

  <p>When querying the database for a view, we may want to provide
     additional query criteria based on the objects associated with
     this view. To support this a view defines query members for all
     the associated objects which allows us to refer to such objects'
     members using the <code>odb::query&lt;view>::member</code> expressions.
     This is similar to how we can refer to object members using the
     <code>odb::query&lt;object>::member</code> expressions when
     querying the database for an object. For example:</p>

  <pre class="cxx">
using query = odb::query&lt;employee_count>;

transaction t (db.begin ());

// Find the number of employees with the Doe last name. Result of this
// aggregate query contains only one element so use the query_value()
// shortcut function.
//
employee_count ec (
  db.query_value&lt;employee_count> (query::last == "Doe"));

cout &lt;&lt; ec.count &lt;&lt; endl;

t.commit ();
  </pre>

  <p>In the above query we used the last name data member from the associated
     <code>employee</code> object to only count employees with the specific
     name.</p>

  <p>When a view has only one associated object, the query members
     corresponding to this object are defined directly in the
     <code>odb::query&lt;view></code> scope. For instance,
     in the above example, we referred to the last name member as
     <code>odb::query&lt;employee_count>::last</code>. However, if
     a view has multiple associated objects, then query members
     corresponding to each such object are defined in a nested
     scope named after the object. As an example, consider
     the <code>employee_employer</code> view again:</p>

  <pre class="cxx">
#pragma db view object(employee) object(employer)
struct employee_employer
{
  std::string first;
  std::string last;

  #pragma db column(employer::name_)
  std::string employer_name;
};
  </pre>

  <p>Now, to refer to the last name data member from the <code>employee</code>
     object we use the
     <code>odb::query&lt;...>::employee::last</code> expression.
     Similarly, to refer to the employer name, we use the
     <code>odb::query&lt;...>::employer::name</code> expression.
     For example:</p>

  <pre class="cxx">
using result = odb::result&lt;employee_employer>;
using query = odb::query&lt;employee_employer>;

transaction t (db.begin ());

result r (db.query&lt;employee_employer> (
  query::employee::last == "Doe" &amp;&amp;
  query::employer::name == "Simple Tech Ltd"));

for (result::iterator i (r.begin ()); i != r.end (); ++i)
  cout &lt;&lt; i->first &lt;&lt; " " &lt;&lt; i->last &lt;&lt;  " " &lt;&lt; i->employer_name &lt;&lt; endl;

t.commit ();
  </pre>

  <p>If we assign an alias to an object, then this alias is used to
     name the query members scope instead of the object name. As an
     example, consider the <code>employee_country</code> view again:</p>

  <pre class="cxx">
#pragma db view object(employee) \
  object(country = res_country: employee::residence_) \
  object(country = nat_country: employee::nationality_)
struct employee_country
{
  ...
};
  </pre>

  <p>And a query which returns all the employees that have the same
     country of residence and nationality:</p>

  <pre class="cxx">
using query = odb::query&lt;employee_country>;
using result = odb::result&lt;employee_country>;

transaction t (db.begin ());

result r (db.query&lt;employee_country> (
  query::res_country::name == query::nat_country::name));

for (result::iterator i (r.begin ()); i != r.end (); ++i)
  cout &lt;&lt; i->first &lt;&lt; " " &lt;&lt; i->last &lt;&lt; " " &lt;&lt; i->res_country_name &lt;&lt; endl;

t.commit ();
  </pre>

  <p>Note also that unlike object query members, view query members do
     no support referencing members in related objects. For example,
     the following query is invalid:</p>

  <pre class="cxx">
using query = odb::query&lt;employee_name>;
using result = odb::result&lt;employee_name>;

transaction t (db.begin ());

result r (db.query&lt;employee_name> (
  query::employed_by->name == "Simple Tech Ltd"));

t.commit ();
  </pre>

  <p>To get this behavior, we would instead need to associate the
     <code>employer</code> object with this view and then use the
     <code>query::employer::name</code> expression instead of
     <code>query::employed_by->name</code>.</p>

  <p>As we have discussed above, if specified, an object alias is
     used instead of the object name in the join condition, data
     member references in the <code>db&nbsp;column</code> pragma,
     as well as to name the query members scope. The object alias
     is also used as a table name alias in the underlying
     <code>SELECT</code> statement generated by the ODB compiler.
     Normally, you would not use the table alias directly with
     object views. However, if for some reason you need to refer
     to a table column directly, for example, as part of a native
     query expression, and you need to qualify the column with
     the table, then you will need to use the table alias instead.</p>

  <h2><a name="10.2">10.2 Object Loading Views</a></h2>

  <p>A special variant of object views is object loading views. Object
     loading views allow us to load one or more complete objects
     instead of, or in addition to, a subset of data member. While we
     can often achieve the same end result by calling
     <code>database::load()</code>, using a view has several advantages.</p>

  <p>If we need to load multiple objects, then using a view allows us
     to do this with a single <code>SELECT</code> statement execution
     instead of one for each object that would be necessary in case of
     <code>load()</code>. A view can also be useful for loading only
     a single object if the query criterion that we would like to use
     involves other, potentially unrelated, objects. We will examine
     concrete examples of these and other scenarios in the rest of this
     section.</p>

  <p>To load a complete object as part of a view we use a data member of
     the pointer to object type, just like for object relationships
     (<a href="#6">Chapter 6, "Relationships"</a>). As an example, here
     is how we can load both the <code>employee</code> and
     <code>employer</code> objects from the previous section with a single
     statement:</p>

  <pre class="cxx">
#pragma db view object(employee) object(employer)
struct employee_employer
{
  shared_ptr&lt;employee> ee;
  shared_ptr&lt;employer> er;
};
  </pre>

  <p>We use an object loading view just like any other view. In the
     result of a query, as we would expect, the pointer data members
     point to the loaded objects. For example:</p>

  <pre class="cxx">
using query = odb::query&lt;employee_employer>;

transaction t (db.begin ());

for (const employee_employer&amp; r:
       db.query&lt;employee_employer> (query::employee::age &lt; 31))
{
  cout &lt;&lt; r.ee->age () &lt;&lt; " " &lt;&lt; r.er->name () &lt;&lt; endl;
}

t.commit ();
  </pre>

  <p>As another example, consider a query that loads the <code>employer</code>
     objects using some condition based on its employees. For instance, we
     want to find all the employers that employ people over 65 years old.
     We can use this object loading view to implement such a query (notice
     the <code>distinct</code> result modifier discussed later in
     <a href="#10.5">Section 10.5, "View Query Conditions"</a>):</p>

  <pre class="cxx">
#pragma db view object(employer) object(employee) query(distinct)
struct employer_view
{
  shared_ptr&lt;employer> er;
};
  </pre>

  <p>And this is how we can use this view to find all the employers that
     employ seniors:</p>

  <pre class="cxx">
using query = odb::query&lt;employer_view>;

db.query&lt;employer_view> (query::employee::age > 65)
  </pre>

  <p>We can even use object loading views to load completely unrelated
     (from the ODB object relationships point of view) objects. For example,
     the following view will load all the employers that are named the
     same as a country (notice the <code>inner</code> join type):</p>

  <pre class="cxx">
#pragma db view object(employer) \
  object(country inner: employer::name == country::name)
struct employer_named_country
{
  shared_ptr&lt;employer> e;
  shared_ptr&lt;country> c;
};
  </pre>

  <p>An object loading view can contain ordinary data members
     in addition to object pointers. For example, if we are only
     interested in the country code in the above view, then we
     can reimplement it like this:</p>

  <pre class="cxx">
#pragma db view object(employer) \
  object(country inner: employer::name == country::name)
struct employer_named_country
{
  shared_ptr&lt;employer> e;
  std::string code;
};
  </pre>

  <p>Object loading views also have a few rules and restrictions.
     Firstly, the pointed-to object in the data member must be associated
     with the view. Furthermore, if the associated object has an alias,
     then the data member name must be the same as the alias (more
     precisely, the public name derived from the data member must
     match the alias; which means we can use normal data member
     decorations such as trailing underscores, etc., see the previous
     section for more information on public names). The following view
     illustrates the use of aliases as data member names:</p>

  <pre class="cxx">
#pragma db view object(employee)               \
  object(country = res: employee::residence_)  \
  object(country = nat: employee::nationality_)
struct employee_country
{
  shared_ptr&lt;country> res;
  shared_ptr&lt;country> nat_;
};
  </pre>

  <p>Finally, the object pointers must be direct data members of
     the view. Using, for example, a composite value that contains
     pointers as a view data member is not supported. Note also
     that depending on the join type you are using, some of the
     resulting pointers might be <code>NULL</code>.</p>

  <p>Up until now we have consistently used <code>shared_ptr</code>
     as an object pointer in our views. Can we use other pointers,
     such as <code>unique_ptr</code> or raw pointers? To answer
     this question we first need to discuss what happens with
     object pointers that may be inside objects that a view
     loads. As a concrete example, let us revisit the
     <code>employee_employer</code> view from the beginning of
     this section:</p>

  <pre class="cxx">
#pragma db view object(employee) object(employer)
struct employee_employer
{
  shared_ptr&lt;employee> ee;
  shared_ptr&lt;employer> er;
};
  </pre>

  <p>This view loads two objects: <code>employee</code> and
     <code>employer</code>. The <code>employee</code> object,
     however, also contains a pointer to <code>employer</code>
     (see the <code>employed_by_</code> data member). In fact,
     this is the same object that the view loads since <code>employer</code>
     is associated with the view using this same relationship (ODB
     automatically uses it since it is the only one). The correct
     result of loading such a view is then clear: both <code>er</code> and
     <code>er->employed_by_</code> must point to (or share) the
     same instance.</p>

  <p>Just like object loading via the <code>database</code> class
     functions, views achieve this correct behavior of only loading
     a single instance of the same object with the help of session's
     object cache (<a href="#11">Chapter 11, "Session"</a>). In fact,
     object loading views enforce this by throwing the
     <code>session_required</code> exception if there is no current
     session and the view loads an object that is also indirectly
     loaded by one of the other objects. The ODB compiler will also
     issue diagnostics if such an object has session support
     disabled (<a href="#14.1.10">Section 14.1.10,
     "<code>session</code>"</a>).</p>

  <p>With this understanding we can now provide the correct implementation
     of our transaction that uses the <code>employee_employer</code> view:</p>

  <pre class="cxx">
using query = odb::query&lt;employee_employer>;

transaction t (db.begin ());
odb::session s;

for (const employee_employer&amp; r:
       db.query&lt;employee_employer> (query::employee::age &lt; 31))
{
  assert (r.ee->employed_by_ == r.er);
  cout &lt;&lt; r.ee->age () &lt;&lt; " " &lt;&lt; r.er->name () &lt;&lt; endl;
}

t.commit ();
  </pre>

  <p>It might seem logical, then, to always load all the objects from
     all the eager relationships with the view. After all, this will
     lead to them all being loaded with a single statement. While
     this is theoretically true, the reality is slightly more nuanced.
     If there is a high probability of the object already have been
     loaded and sitting in the cache, then not loading the object
     as part of the view (and therefore not fetching all its data
     from the database) might result in better performance.</p>

  <p>Now we can also answer the question about which pointers we can
     use in object loading views. From the above discussion it should
     be clear that if an object that we are loading is also part of a
     relationship inside another object that we are loading, then we
     should use some form of a shared ownership pointer. If, however,
     there are no relationships involved, as is the case, for example,
     in our <code>employer_named_country</code> and
     <code>employee_country</code> views above, then we can use a
     unique ownership pointer such as <code>unique_ptr</code>.</p>

  <p>Note also that your choice of a pointer type can be limited by the
     "official" object pointer type assigned to the object
     (<a href="#3.3">Section 3.3, "Object and View Pointers"</a>).
     For example, if the object pointer type is <code>shared_ptr</code>,
     you will not be able to use <code>unique_ptr</code> to load
     such an object into a view since initializing <code>unique_ptr</code>
     from <code>shared_ptr</code> would be a mistake.</p>

  <p>Unless you want to perform your own object cleanup, raw object
     pointers in views are not particularly useful. They do have one
     special semantics, however: If a raw pointer is used as a view
     member, then, before creating a new instance, the implementation
     will check if the member is <code>NULL</code>. If it is not, then
     it is assumed to point to an existing instance and the implementation
     will load the data into it instead of creating a new one. The
     primary use of this special functionality is to implement by-value
     loading with the ability to detect <code>NULL</code> values.</p>

  <p>To illustrate this functionality, consider the following view that
     load the employee's residence country by value:</p>

  <pre class="cxx">
#pragma db view object(employee) \
  object(country = res: employee::residence_) transient
struct employee_res_country
{
  using country_ptr = country*;

  #pragma db member(res_) virtual(country_ptr) get(&amp;this.res) \
    set(this.res_null = ((?) == nullptr))

  country res;
  bool res_null;
};
  </pre>

  <p>Here we are using a virtual data member
     (<a href="#14.4.13">Section 14.4.13, "<code>virtual</code>"</a>) to
     add an object pointer member to the view. Its accessor expression
     returns the pointer to the <code>res</code> member so that
     the implementation can load the data into it. The modifier
     expression checks the passed pointer to initialize the
     <code>NULL</code> value indicator. Here, the two possible
     values that can be passed to the modifier expression are
     the address of the <code>res</code> member that we returned
     earlier from the accessor and <code>NULL</code> (strictly
     speaking, there is a third possibility: the address of an
     object that was found in the session cache).</p>

  <p>If we are not interested in the <code>NULL</code> indicator,
     then the above view can simplified to this:</p>

  <pre class="cxx">
#pragma db view object(employee) \
  object(country = res: employee::residence_) transient
struct employee_res_country
{
  using country_ptr = country*;

  #pragma db member(res_) virtual(country_ptr) get(&amp;this.res) set()

  country res;
};
  </pre>

  <p>That is, we specify an empty modifier expression which leads to
     the value being ignored.</p>

  <p>As another example of by-value loading, consider a view that allows
     us to load objects into existing instances that have been allocated
     outside the view:</p>

  <pre class="cxx">
#pragma db view object(employee)               \
  object(country = res: employee::residence_)  \
  object(country = nat: employee::nationality_)
struct employee_country
{
  employee_country (country&amp; r, country&amp; n): res (&amp;r), nat (&amp;n) {}

  country* res;
  country* nat;
};
  </pre>

  <p>And here is how we can use this view:</p>

  <pre class="cxx">
using result = odb::result&lt;employee_country>;

transaction t (db.begin ());

result r (db.query&lt;employee_country> (...);

for (result::iterator i (r.begin ()); i != r.end (); ++i)
{
  country res, nat;
  employee_country v (res, nat);
  i.load (v);

  if (v.res != nullptr)
    ... // Result is in res.

  if (v.nat != nullptr)
    ... // Result is in nat.
}

t.commit ();
  </pre>

  <p>As a final example of the by-value loading, consider the following
     view which implements a slightly more advanced logic: if the object
     is already in the session cache, then it sets the pointer data member
     in the view (<code>er_p</code>) to that. Otherwise, it loads the data
     into the by-value instance (<code>er</code>). We can also check
     whether the pointer data member points to the instance to distinguish
     between the two outcomes. And we can check it for <code>nullptr</code>
     to detect <code>NULL</code> values.</p>

  <pre class="cxx">
#pragma db view object(employer)
struct employer_view
{
  // Since we may be getting the pointer as both smart and raw, we
  // need to create a bit of support code to use in the modifier
  // expression.
  //
  void set_er (employer* p) {er_p = p;}                   // &amp;er or NULL.
  void set_er (shared_ptr&lt;employer> p) {er_p = p.get ();} // From cache.

  #pragma db get(&amp;this.er) set(set_er(?))
  employer* er_p;

  #pragma db transient
  employer er;

  // Return-by-value support (e.g., query_value()).
  //
  employer_view (): er_p (0) {}
  employer_view (const employer_view&amp; x)
    : er_p (x.er_p == &amp;x.er ? &amp;er : x.er_p), er (x.er) {}
};
  </pre>

  <p>We can use object loading views with polymorphic objects
     (<a href="#8.2">Section 8.2, "Polymorphism Inheritance"</a>). Note,
     however, that when loading a derived object via the base pointer
     in a view, a separate statement will be executed to load the
     dynamic part of the object. There is no support for by-value
     loading for polymorphic objects.</p>

  <p>We can also use object loading views with objects without id
     (<a href="#14.1.6">Section 14.1.6, "<code>no_id</code>"</a>).
     Note, however, that for such objects, <code>NULL</code> values
     are not automatically detected (since there is no primary key,
     which is otherwise guaranteed to be not <code>NULL</code>, there
     might not be a column on which to base this detection). The
     workaround for this limitation is to load an otherwise not
     <code>NULL</code> column next to the object which will serve
     as an indicator. For example:</p>

  <pre class="cxx">
#pragma db object no_id
class object
{
  ...

  int n; // NOT NULL
  std::string s;
};

#include &lt;odb/nullable.hxx>

#pragma db view object(object)
struct view
{

  odb::nullable&lt;int> n; // If 'n' is NULL, then, logically, so is 'o'.
  unique_ptr&lt;object> o;
};
  </pre>

  <h2><a name="10.3">10.3 Table Views</a></h2>

  <p>A table view is similar to an object view except that it is
     based on one or more database tables instead of persistent
     objects. Table views are primarily useful when dealing with
     ad-hoc tables that are not mapped to persistent classes.</p>

  <p>To associate one or more tables with a view we use the
     <code>db&nbsp;table</code> pragma (<a href="#14.2.2">Section 14.2.2,
     "<code>table</code>"</a>). To associate the second and subsequent
     tables we repeat the <code>db&nbsp;table</code> pragma for each
     additional table. For example, the following view is based on the
     <code>employee_extra</code> legacy table we have defined at the
     beginning of the chapter.</p>

  <pre class="cxx">
#pragma db view table("employee_extra")
struct employee_vacation
{
  #pragma db column("employee_id") type("INTEGER")
  unsigned long long employee_id;

  #pragma db column("vacation_days") type("INTEGER")
  unsigned short vacation_days;
};
  </pre>

  <p>Besides the table name in the <code>db&nbsp;table</code> pragma
     we also have to specify the column name for each view data
     member. Note that unlike for object views, the ODB compiler
     does not try to automatically come up with column names for
     table views. Furthermore, we cannot use references to object
     members either, since there are no associated objects in table
     views. Instead, the actual column name or column expression
     must be specified as a string literal. The column name can
     also be qualified with a table name either in the
     <code>"table.column"</code> form or, if either a table
     or a column name contains a period, in the
     <code>"table"."column"</code> form. The following example
     illustrates the use of a column expression:</p>

  <pre class="cxx">
#pragma db view table("employee_extra")
struct employee_max_vacation
{
  #pragma db column("max(vacation_days)") type("INTEGER")
  unsigned short max_vacation_days;
};
  </pre>

  <p>Both the associated table names and the column names can be qualified
     with a database schema, for example:</p>

  <pre class="cxx">
#pragma db view table("hr.employee_extra")
struct employee_max_vacation
{
  #pragma db column("hr.employee_extra.vacation_days") type("INTEGER")
  unsigned short vacation_days;
};
  </pre>

  <p>For more information on database schemas and the format of the
     qualified names, refer to <a href="#14.1.8">Section 14.1.8,
     "<code>schema</code>"</a>.</p>

  <p>Note also that in the above examples we specified the SQL type
     for each of the columns to make sure that the ODB compiler
     has knowledge of the actual types as specified in the database
     schema. This is required to obtain correct and optimal
     generated code.</p>


  <p>The complete syntax of the <code>db&nbsp;table</code> pragma
     is similar to the <code>db&nbsp;object</code> pragma and is shown
     below:</p>

  <p><code><b>table("</b><i>name</i><b>"</b>
                    [<b>=</b> <b>"</b><i>alias</i><b>"</b>]
                    [<i>join-type</i>]
                    [<b>:</b> <i>join-condition</i>]<b>)</b></code></p>

  <p>The <i>name</i> part is a database table name. The optional
     <i>alias</i> part gives this table an alias. If provided, the
     alias must be used instead of the table whenever a reference
     to a table is used. Contexts where such a reference may
     be needed include the join condition (discussed below),
     column names, and query expressions. The optional <i>join-type</i>
     part specifies the way this table is associated. It can
     be <code>left</code>, <code>right</code>, <code>full</code>,
     <code>inner</code>, and <code>cross</code> with <code>left</code>
     being the default. Finally, the optional <i>join-condition</i>
     part provides the criteria which should be used to associate this
     table with any of the previously associated tables or, as we will see in
     <a href="#10.4">Section 10.4, "Mixed Views"</a>, objects. Note that
     while the first associated table can have an alias, it cannot have
     a join type or condition.</p>

  <p>Similar to object views, for each subsequent associated table the
     ODB compiler needs a join condition. However, unlike for object views,
     for table views the ODB compiler does not try to come up with one
     automatically. Furthermore, we cannot use references to object
     members corresponding to object relationships either, since there
     are no associated objects in table views. Instead, for each
     subsequent associated table, a join condition must be
     specified as a custom query expression. While the syntax of the
     query expression is the same as in the query facility used to query
     the database for objects (<a href="#4">Chapter 4, "Querying the
     Database"</a>), a join condition for a table is normally specified
     as a single string literal containing a native SQL query expression.</p>

  <p>As an example of a multi-table view, consider the
     <code>employee_health</code> table that we define in addition
     to <code>employee_extra</code>:</p>

  <pre class="sql">
CREATE TABLE employee_health(
  employee_id INTEGER NOT NULL,
  sick_leave_days INTEGER NOT NULL)
  </pre>

  <p>Given these two tables we can now define a view that returns both
     the vacation and sick leave information for each employee:</p>

  <pre class="cxx">
#pragma db view table("employee_extra" = "extra") \
  table("employee_health" = "health": \
        "extra.employee_id = health.employee_id")
struct employee_leave
{
  #pragma db column("extra.employee_id") type("INTEGER")
  unsigned long long employee_id;

  #pragma db column("vacation_days") type("INTEGER")
  unsigned short vacation_days;

  #pragma db column("sick_leave_days") type("INTEGER")
  unsigned short sick_leave_days;
};
  </pre>

  <p>Querying the database for a table view is the same as for an
     object view except that we can only use native query expressions.
     For example:</p>

  <pre class="cxx">
using query = odb::query&lt;employee_leave>;
using result = odb::result&lt;employee_leave>;

transaction t (db.begin ());

unsigned short v_min = ...
unsigned short l_min = ...

result r (db.query&lt;employee_leave> (
  "vacation_days > " + query::_val(v_min) + "AND" +
  "sick_leave_days > " + query::_val(l_min)));

t.commit ();
  </pre>


  <h2><a name="10.4">10.4 Mixed Views</a></h2>

  <p>A mixed view has both associated objects and tables. As a first
     example of a mixed view, let us improve <code>employee_vacation</code>
     from the previous section to return the employee's first
     and last names instead of the employee id. To achieve this we
     have to associate both the <code>employee</code> object and
     the <code>employee_extra</code> table with the view:</p>

  <pre class="cxx">
#pragma db view object(employee) \
  table("employee_extra" = "extra": "extra.employee_id = " + employee::id_)
struct employee_vacation
{
  std::string first;
  std::string last;

  #pragma db column("extra.vacation_days") type("INTEGER")
  unsigned short vacation_days;
};
  </pre>

  <p>When querying the database for a mixed view, we can use query members
     for the parts of the query expression that involves object members
     but have to fall back to using the native syntax for the parts that
     involve table columns. For example:</p>

  <pre class="cxx">
using query = odb::query&lt;employee_vacation>;
using result = odb::result&lt;employee_vacation>;

transaction t (db.begin ());

result r (db.query&lt;employee_vacation> (
  (query::last == "Doe") + "AND extra.vacation_days &lt;> 0"));

for (result::iterator i (r.begin ()); i != r.end (); ++i)
  cout &lt;&lt; i->first &lt;&lt; " " &lt;&lt; i->last &lt;&lt; " " &lt;&lt; i->vacation_days &lt;&lt; endl;

t.commit ();
  </pre>

  <p>As another example, consider a more advanced view that associates
     two objects via a legacy table. This view allows us to find the
     previous employer name for each employee:</p>

  <pre class="cxx">
#pragma db view object(employee) \
  table("employee_extra" = "extra": "extra.employee_id = " + employee::id_) \
  object(employer: "extra.previous_employer_id = " + employer::id_)
struct employee_prev_employer
{
  std::string first;
  std::string last;

  // If previous_employer_id is NULL, then the name will be NULL as well.
  // We use the odb::nullable wrapper to handle this.
  //
  #pragma db column(employer::name_)
  odb::nullable&lt;std::string> prev_employer_name;
};
  </pre>

  <h2><a name="10.5">10.5 View Query Conditions</a></h2>

  <p>Object, table, and mixed views can also specify an optional query
     condition that should be used whenever the database is queried for
     this view. To specify a query condition we use the
     <code>db&nbsp;query</code> pragma (<a href="#14.2.3">Section 14.2.3,
     "<code>query</code>"</a>).</p>

  <p>As an example, consider a view that returns some information about
     all the employees that are over a predefined retirement age.
     One way to implement this would be to define a standard object
     view as we have done in the previous sections and then use a
     query like this:</p>

  <pre class="cxx">
result r (db.query&lt;employee_retirement> (query::age > 50));
  </pre>

  <p>The problem with the above approach is that we have to keep
     repeating the <code>query::age > 50</code> expression every
     time we execute the query, even though this expression always
     stays the same. View query conditions allow us to solve this
     problem. For example:</p>

  <pre class="cxx">
#pragma db view object(employee) query(employee::age > 50)
struct employee_retirement
{
  std::string first;
  std::string last;
  unsigned short age;
};
  </pre>

  <p>With this improvement we can rewrite our query like this:</p>

  <pre class="cxx">
result r (db.query&lt;employee_retirement> ());
  </pre>

   <p>But what if we may also need to restrict the result set based on
      some varying criteria, such as the employee's last name? Or, in other
      words, we may need to combine a constant query expression specified
      in the <code>db&nbsp;query</code> pragma with the varying expression
      specified at the query execution time. To allow this, the
      <code>db&nbsp;query</code> pragma syntax supports the use of the special
      <code>(?)</code> placeholder that indicates the position in the
      constant query expression where the runtime expression should be
      inserted. For example:</p>

  <pre class="cxx">
#pragma db view object(employee) query(employee::age > 50 &amp;&amp; (?))
struct employee_retirement
{
  std::string first;
  std::string last;
  unsigned short name;
};
  </pre>

  <p>With this change we can now use additional query criteria in our
     view:</p>

  <pre class="cxx">
result r (db.query&lt;employee_retirement> (query::last == "Doe"));
  </pre>

  <p>The syntax of the expression in a query condition is the same as in
     the query facility used to query the database for objects
     (<a href="#4">Chapter 4, "Querying the Database"</a>) except for
     two differences. Firstly, for query members, instead of
     using <code>odb::query&lt;object>::member</code> names, we refer
     directly to object members, using the object alias instead of the
     object name if an alias was assigned. Secondly, query conditions
     support the special <code>(?)</code> placeholder which can be used
     both in the C++-integrated query expressions as was shown above
     and in native SQL expressions specified as string literals. The
     following view is an example of the latter case:</p>

  <pre class="cxx">
#pragma db view table("employee_extra") \
  query("vacation_days &lt;> 0 AND (?)")
struct employee_vacation
{
  ...
};
  </pre>

  <p>Another common use case for query conditions are views with the
     <code>ORDER BY</code> or <code>GROUP BY</code> clause. Such
     clauses are normally present in the same form in every query
     involving such views. As an example, consider an aggregate
     view which calculate the minimum and maximum ages of employees
     for each employer:</p>

  <pre class="cxx">
#pragma db view object(employee) object(employer) \
  query((?) + "GROUP BY" + employer::name_)
struct employer_age
{
  #pragma db column(employer::name_)
  std::string employer_name;

  #pragma db column("min(" + employee::age_ + ")")
  unsigned short min_age;

  #pragma db column("max(" + employee::age_ + ")")
  unsigned short max_age;
};
  </pre>

  <p>The query condition can be optionally followed (or replaced,
     if no constant query expression is needed) by one or more
     <em>result modifiers</em>. Currently supported result modifiers
     are <code>distinct</code> (which is translated to <code>SELECT
     DISTINCT</code>) and <code>for_update</code> (which is translated
     to <code>FOR UPDATE</code> or equivalent for database systems
     that support it). As an example, consider a view that
     allows us to get some information about employers ordered
     by the object id and without any duplicates:</p>

  <pre class="cxx">
#pragma db view object(employer) object(employee) \
  query((?) + "ORDER BY" + employer::name_, distinct)
struct employer_info
{
  ...
};
  </pre>

  <p>If we don't require ordering, then this view can be re-implemented
     like this:</p>

  <pre class="cxx">
#pragma db view object(employer) object(employee) query(distinct)
struct employer_info
{
  ...
};
  </pre>

  <h2><a name="10.6">10.6 Native Views</a></h2>

  <p>The last kind of view supported by ODB is a native view. Native
     views are a low-level mechanism for capturing results of native
     SQL queries, stored procedure calls, etc. Native views don't have
     associated tables or objects. Instead, we use the
     <code>db&nbsp;query</code> pragma to specify the native SQL query,
     which should normally include the select-list and, if applicable,
     the from-list. For example, here is how we can re-implement the
     <code>employee_vacation</code> table view from Section 10.3 above
     as a native view:</p>

  <pre class="cxx">
#pragma db view query("SELECT employee_id, vacation_days " \
                      "FROM employee_extra")
struct employee_vacation
{
  #pragma db type("INTEGER")
  unsigned long long employee_id;

  #pragma db type("INTEGER")
  unsigned short vacation_days;
};
  </pre>

  <p>In native views the columns in the query select-list are
     associated with the view data members in the order specified.
     That is, the first column is stored in the first member, the
     second column &mdash; in the second member, and so on. The ODB compiler
     does not perform any error checking in this association. As a result
     you must make sure that the number and order of columns in the
     query select-list match the number and order of data members
     in the view. This is also the reason why we are not
     required to provide the column name for each data member in native
     views, as is the case for object and table views.</p>

  <p>Note also that while it is always possible to implement a table
     view as a native view, the table views must be preferred since
     they are safer. In a native view, if you add, remove, or
     rearrange data members without updating the column list in the
     query, or vice versa, at best, this will result in a runtime
     error. In contrast, in a table view such changes will result
     in the query being automatically updated.</p>

  <p>Similar to object and table views, the query specified for
     a native view can contain the special <code>(?)</code>
     placeholder which is replaced with the query expression
     specified at the query execution time.
     If the native query does not contain a placeholder, as in
     the example above, then any query expression specified at
     the query execution time is appended to the query text
     along with the <code>WHERE</code> keyword, if required.
     The following example shows the usage of the placeholder:</p>

  <pre class="cxx">
#pragma db view query("SELECT employee_id, vacation_days " \
                      "FROM employee_extra " \
                      "WHERE vacation_days &lt;> 0 AND (?)")
struct employee_vacation
{
  ...
};
  </pre>

  <p>As another example, consider a view that returns the next
     value of a database sequence:</p>

  <pre class="cxx">
#pragma db view query("SELECT nextval('my_seq')")
struct sequence_value
{
  unsigned long long value;
};
  </pre>

  <p>While this implementation can be acceptable in some cases, it has
     a number of drawbacks. Firstly, the name of the sequence is
     fixed in the view, which means if we have a second sequence, we
     will have to define another, almost identical view. Similarly,
     the operation that we perform on the sequence is also fixed.
     In some situations, instead of returning the next value, we may
     need the last value.</p>

  <p>Note that we cannot use the placeholder mechanism to resolve
     these problems since placeholders can only be used in the
     <code>WHERE</code>, <code>GROUP BY</code>, and similar
     clauses. In other words, the following won't work:</p>

  <pre class="cxx">
#pragma db view query("SELECT nextval('(?)')")
struct sequence_value
{
  unsigned long long value;
};

result r (db.query&lt;sequence_value> ("my_seq"));
  </pre>

  <p>To support these kinds of use cases, ODB allows us to specify the
     complete query for a native view at runtime rather than at the view
     definition. To indicate that a native view has a runtime query,
     we can either specify the empty <code>db&nbsp;query</code>
     pragma or omit the pragma altogether. For example:</p>

  <pre class="cxx">
#pragma db view
struct sequence_value
{
  unsigned long long value;
};
  </pre>

  <p>Given this view, we can perform the following queries:</p>

  <pre class="cxx">
using query = odb::query&lt;sequence_value>;
using result = odb::result&lt;sequence_value>;

string seq_name = ...

result l (db.query&lt;sequence_value> (
  "SELECT lastval('" + seq_name + "')"));

result n (db.query&lt;sequence_value> (
  "SELECT nextval('" + seq_name + "')"));
  </pre>

   <p>Native views can also be used to call and handle results of
      stored procedures. The semantics and limitations of stored
      procedures vary greatly between database systems while some
      do not support this functionality at all. As a result, support
      for calling stored procedures using native views is described
      for each database system in <a href="#II">Part II, "Database
      Systems"</a>.</p>

  <h2><a name="10.7">10.7 Other View Features and Limitations</a></h2>

  <p>Views cannot be derived from other views. However, you can derive
     a view from a transient C++ class. View data members cannot be
     object pointers. If you need to access data from a pointed-to
     object, then you will need to associate such an object with
     the view. Similarly, view data members cannot be containers.
     These two limitations also apply to composite value types that
     contain object pointers or containers. Such composite values
     cannot be used as view data members.</p>

  <p>On the other hand, composite values that do not contain object
     pointers or containers can be used in views. As an example,
     consider a modified version of the <code>employee</code> persistent
     class that stores a person's name as a composite value:</p>

  <pre class="cxx">
#pragma db value
class person_name
{
  std::string first_;
  std::string last_;
};

#pragma db object
class employee
{
  ...

  person_name name_;

  ...
};
  </pre>

  <p>Given this change, we can re-implement the <code>employee_name</code>
     view like this:</p>

  <pre class="cxx">
#pragma db view object(employee)
struct employee_name
{
  person_name name;
};
  </pre>

  <p>It is also possible to extract some or all of the nested members
     of a composite value into individual view data members. Here is
     how we could have defined the <code>employee_name</code> view
     if we wanted to keep its original structure:</p>

  <pre class="cxx">
#pragma db view object(employee)
struct employee_name
{
  #pragma db column(employee::name.first_)
  std::string first;

  #pragma db column(employee::name.last_)
  std::string last;
};
  </pre>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="11">11 Session</a></h1>

  <p>A session is an application's unit of work that may encompass several
     database transactions. In this version of ODB a session is just an
     object cache. In future versions it may provide additional
     functionality, such as delayed database operations and automatic
     object state change tracking. As discussed later in
     <a href="#11.2">Section 11.2, "Custom Sessions"</a>, it is also
     possible to provide a custom session implementation that provides
     these or other features.</p>

  <p>Session support is optional and can be enabled or disabled on the
     per object basis using the <code>db&nbsp;session</code> pragma, for
     example:</p>

  <pre class="cxx">
#pragma db object session
class person
{
  ...
};
  </pre>

  <p>We can also enable or disable session support for a group of
     objects at the namespace level:</p>

  <pre class="cxx">
#pragma db namespace session
namespace accounting
{
  #pragma db object                // Session support is enabled.
  class employee
  {
    ...
  };

  #pragma db object session(false) // Session support is disabled.
  class employer
  {
    ...
  };
}
  </pre>

  <p>Finally, we can pass the <code>--generate-session</code> ODB compiler
     option to enable session support by default. With this option session
     support will be enabled for all the persistent classes except those
     for which it was explicitly disabled using the
     <code>db&nbsp;session</code>. An alternative to this method with the
     same effect is to enable session support for the global namespace:</p>

  <pre class="cxx">
#pragma db namespace() session
  </pre>

  <p>Each thread of execution in an application can have only one active
     session at a time. A session is started by creating an instance of
     the <code>odb::session</code> class and is automatically terminated
     when this instance is destroyed. You will need to include the
     <code>&lt;odb/session.hxx></code> header file to make this class
     available in your application. For example:</p>

  <pre class="cxx">
#include &lt;odb/database.hxx>
#include &lt;odb/session.hxx>
#include &lt;odb/transaction.hxx>

using namespace odb::core;

{
  session s;

  // First transaction.
  //
  {
    transaction t (db.begin ());
    ...
    t.commit ();
  }

  // Second transaction.
  //
  {
    transaction t (db.begin ());
    ...
    t.commit ();
  }

  // Session 's' is terminated here.
}
  </pre>

  <p>The <code>session</code> class has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  class session
  {
  public:
    session (bool make_current = true);
    ~session ();

    // Copying or assignment of sessions is not supported.
    //
  private:
    session (const session&amp;);
    session&amp; operator= (const session&amp;);

    // Current session interface.
    //
  public:
    static session&amp;
    current ();

    static bool
    has_current ();

    static void
    current (session&amp;);

    static void
    reset_current ();

    static session*
    current_pointer ();

    static void
    current_pointer (session*);

    // Object cache interface.
    //
  public:
    template &lt;typename T>
    struct cache_position {...};

    template &lt;typename T>
    cache_position&lt;T>
    cache_insert (database&amp;,
                  const object_traits&lt;T>::id_type&amp;,
                  const object_traits&lt;T>::pointer_type&amp;);

    template &lt;typename T>
    object_traits&lt;T>::pointer_type
    cache_find (database&amp;, const object_traits&lt;T>::id_type&amp;) const;

    template &lt;typename T>
    void
    cache_erase (const cache_position&lt;T>&amp;);

    template &lt;typename T>
    void
    cache_erase (database&amp;, const object_traits&lt;T>::id_type&amp;);
  };
}
  </pre>

  <p>The session constructor creates a new session and, if the
     <code>make_current</code> argument is <code>true</code>, sets it as a
     current session for this thread. If we try to make a session current
     while there is already another session in effect for this thread,
     then the constructor throws the <code>odb::already_in_session</code>
     exception. The destructor clears the current session for this
     thread if this session is the current one.</p>

  <p>The static <code>current()</code> accessor returns the currently active
     session for this thread. If there is no active session, this function
     throws the <code>odb::not_in_session</code> exception. We can check
     whether there is a session in effect in this thread using the
     <code>has_current()</code> static function.</p>

  <p>The static <code>current()</code> modifier allows us to set the
     current session for this thread. The <code>reset_current()</code>
     static function clears the current session. These two functions
     allow for more advanced use cases, such as multiplexing
     two or more sessions on the same thread.</p>

  <p>The static <code>current_pointer()</code> overloaded functions
     provided the same functionality but using pointers. Specifically,
     the <code>current_pointer()</code> accessor can be used to
     test whether there is a current session and get a pointer to it
     all with a single call.</p>

  <p>We normally don't use the object cache interface directly. However,
     it could be useful in some cases, for example, to find out whether
     an object has already been loaded. Note that when calling
     <code>cache_insert()</code>, <code>cache_find()</code>, or
     the second version of <code>cache_erase()</code>, you need to
     specify the template argument (object type) explicitly. It is
     also possible to access the underlying cache data structures
     directly. This can be useful if, for example, you want to
     iterate over the objects store in the cache. Refer to the ODB
     runtime header files for more details on this direct access.</p>

  <h2><a name="11.1">11.1 Object Cache</a></h2>

  <p>A session is an object cache. Every time a session-enabled object is
     made persistent by calling the <code>database::persist()</code> function
     (<a href="#3.8">Section 3.8, "Making Objects Persistent"</a>), loaded
     by calling the <code>database::load()</code> or
     <code>database::find()</code> function (the pointer-returning overloads
     only; <a href="#3.9">Section 3.9, "Loading Persistent Objects"</a>),
     or loaded by iterating over a query result (<a href="#4.4">Section 4.4,
     "Query Result"</a>), the pointer to the persistent object, in the form
     of the canonical object pointer (<a href="#3.3">Section 3.3, "Object
     and View Pointers"</a>), is stored in the session. For as long as the
     session is in effect, any subsequent calls to load the same object will
     return the cached instance. When an object's state is deleted from the
     database with the <code>database::erase()</code> function
     (<a href="#3.11">Section 3.11, "Deleting Persistent Objects"</a>), the
     cached object pointer is removed from the session. For example:</p>

  <pre class="cxx">
shared_ptr&lt;person> p (new person ("John", "Doe"));

session s;
transaction t (db.begin ());

unsigned long long id (db.persist (p));             // p is cached in s.
shared_ptr&lt;person> p1 (db.load&lt;person> (id)); // p1 same as p.

t.commit ();
  </pre>


  <p>The per-object caching policies depend on the object pointer kind
     (<a href="#6.5">Section 6.5, "Using Custom Smart Pointers"</a>).
     Objects with a unique pointer, such as <code>std::auto_ptr</code>
     or <code>std::unique_ptr</code>, as an object pointer are never
     cached since it is not possible to have two such pointers pointing
     to the same object. When an object is persisted via a pointer or
     loaded as a dynamically allocated instance, objects with both raw
     and shared pointers as object pointers are cached. If an object is
     persisted as a reference or loaded into a pre-allocated instance,
     the object is only cached if its object pointer is a raw pointer.</p>

  <p>Also note that when we persist an object as a constant reference
     or constant pointer, the session caches such an object as
     unrestricted (non-<code>const</code>). This can lead to undefined
     behavior if the object being persisted was actually created as
     <code>const</code> and is later found in the session cache and
     used as non-<code>const</code>. As a result, when using sessions,
     it is recommended that all persistent objects be created as
     non-<code>const</code> instances. The following code fragment
     illustrates this point:</p>

  <pre class="cxx">
void save (database&amp; db, shared_ptr&lt;const person> p)
{
  transaction t (db.begin ());
  db.persist (p); // Persisted as const pointer.
  t.commit ();
}

session s;

shared_ptr&lt;const person> p1 (new const person ("John", "Doe"));
unsigned long long id1 (save (db, p1)); // p1 is cached in s as non-const.

{
  transaction t (db.begin ());
  shared_ptr&lt;person> p (db.load&lt;person> (id1)); // p == p1
  p->age (30); // Undefined behavior since p1 was created const.
  t.commit ();
}

shared_ptr&lt;const person> p2 (new person ("Jane", "Doe"));
unsigned long long id2 (save (db, p2)); // p2 is cached in s as non-const.

{
  transaction t (db.begin ());
  shared_ptr&lt;person> p (db.load&lt;person> (id2)); // p == p2
  p->age (30); // Ok, since p2 was not created const.
  t.commit ();
}
  </pre>

  <h2><a name="11.2">11.2 Custom Sessions</a></h2>

  <p>ODB can use a custom session implementation instead of the
     default <code>odb::session</code>. There could be multiple
     reasons for an application to provide its own session. For
     example, the application may already include a notion of an
     object cache or registry which ODB can re-use. A custom
     session can also provide additional functionality, such as
     automatic change tracking, delayed database operations, or
     object eviction. Finally, the session-per-thread approach used
     by <code>odb::session</code> may not be suitable for all
     applications. For instance, some may need a thread-safe
     session that can be shared among multiple threads. For
     an example of a custom session that implements automatic
     change tracking by keeping original copies of the objects,
     refer to the <code>common/session/custom</code> test
     in the <code>odb-tests</code> package.</p>

  <p>To use a custom session we need to specify its type with
     the <code>--session-type</code> ODB compiler command line
     option. We also need to include its definition into the
     generated header file. This can be achieved with the
     <code>--hxx-prologue</code> option. For example, if our
     custom session is called <code>app::session</code> and
     is defined in the <code>app/session.hxx</code> header
     file, then the corresponding ODB compiler options would
     look like this:</p>

  <pre class="terminal">
odb --hxx-prologue "#include \"app/session.hxx\"" \
--session-type ::app::session ...
  </pre>

  <p>A custom session should provide the following interface:</p>

  <pre class="cxx">
class custom_session
{
public:
  static bool
  _has_cache ();

  // Cache management functions.
  //
  template &lt;typename T>
  struct cache_position
  {
    ...
  };

  template &lt;typename T>
  static cache_position&lt;T>
  _cache_insert (odb::database&amp;,
                 const typename odb::object_traits&lt;T>::id_type&amp;,
                 const typename odb::object_traits&lt;T>::pointer_type&amp;);

  template &lt;typename T>
  static typename odb::object_traits&lt;T>::pointer_type
  _cache_find (odb::database&amp;,
               const typename odb::object_traits&lt;T>::id_type&amp;);

  template &lt;typename T>
  static void
  _cache_erase (const cache_position&lt;T>&amp;);

  // Notification functions.
  //
  template &lt;typename T>
  static void
  _cache_persist (const cache_position&lt;T>&amp;);

  template &lt;typename T>
  static void
  _cache_load (const cache_position&lt;T>&amp;);

  template &lt;typename T>
  static void
  _cache_update (odb::database&amp;, const T&amp; obj);

  template &lt;typename T>
  static void
  _cache_erase (odb::database&amp;,
                const typename odb::object_traits&lt;T>::id_type&amp;);
};
  </pre>

  <p>The <code>_has_cache()</code> function shall return <code>true</code>
     if the object cache is in effect in the current thread.</p>

  <p>The <code>cache_position</code> class template represents a position
     in the cache of the inserted object. It should be default and
     copy-constructible as well as copy-assignable. The default
     constructor shall create a special empty/<code>NULL</code>
     position. A call of any of the cache management or notification
     functions with such an empty/<code>NULL</code> position shall be
     ignored.</p>

  <p>The <code>_cache_insert()</code> function shall add the object into
     the object cache and return its position. The <code>_cache_find()</code>
     function looks an object up in the object cache given its id.
     It returns a <code>NULL</code> pointer if the object is not
     found. The <code>_cache_erase()</code> cache management function
     shall remove the object from the cache. It is called
     if the database operation that caused the object to be inserted
     (for example, load) failed. Note also that after insertion the object
     state is undefined. You can only access the object state
     (for example, make a copy or clear a flag) from one of the
     notification functions discussed below.</p>

  <p>The notification functions are called after an object has
     been persisted, loaded, updated, or erased, respectively. If
     your session implementation does not need some of the
     notifications, you still have to provide their functions,
     however, you can leave their implementations empty.</p>

  <p>Notice also that all the cache management and notification
     functions are static. This is done in order to allow for a
     custom notion of a current session. Normally, the first
     step a non-empty implementation will perform is lookup the
     current session.</p>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="12">12 Optimistic Concurrency</a></h1>

  <p>The ODB transaction model (<a href="#3.5">Section 3.5,
     "Transactions"</a>) guarantees consistency as long as we perform all the
     database operations corresponding to a specific application transaction
     in a single database transaction. That is, if we load an object within a
     database transaction and update it in the same transaction, then we are
     guaranteed that the object state that we are updating in the database is
     exactly the same as the state we have loaded. In other words, it is
     impossible for another process or thread to modify the object state
     in the database between these load and update operations.</p>

  <p>In this chapter we use the term <em>application transaction</em>
     to refer to a set of operations on persistent objects that an
     application needs to perform in order to implement some
     application-specific functionality. The term <em>database
     transaction</em> refers to the set of database operations
     performed between the ODB <code>begin()</code> and <code>commit()</code>
     calls. Up until now we have treated application transactions and
     database transactions as essentially the same thing.</p>

  <p>While this model is easy to understand and straightforward to use,
     it may not be suitable for applications that have long application
     transactions. The canonical example of such a situation is an
     application transaction that requires user input between loading
     an object and updating it. Such an operation may take an arbitrary
     long time to complete and performing it within a single database
     transaction will consume database resources as well as prevent
     other processes/threads from updating the object for too long.</p>

  <p>The solution to this problem is to break up the long-lived
     application transaction into several short-lived database
     transactions. In our example that would mean loading the object
     in one database transaction, waiting for user input, and then
     updating the object in another database transaction. For example:</p>

  <pre class="cxx">
unsigned long long id = ...;
person p;

{
  transaction t (db.begin ());
  db.load (id, p);
  t.commit ();
}

cerr &lt;&lt; "enter age for " &lt;&lt; p.first () &lt;&lt; " " &lt;&lt; p.last () &lt;&lt; endl;
unsigned short age;
cin >> age;
p.age (age);

{
  transaction t (db.begin ());
  db.update (p);
  t.commit ();
}
  </pre>

  <p>This approach works well if we only have one process/thread that can ever
     update the object. However, if we have multiple processes/threads
     modifying the same object, then this approach does not guarantee
     consistency anymore. Consider what happens in the above example if
     another process updates the person's last name while we are waiting for
     the user input. Since we loaded the object before this change occured,
     our version of the person's data will still have the old name. Once we
     receive the input from the user, we go ahead and update the object,
     overwriting both the old age with the new one (correct) and the new name
     with the old one (incorrect).</p>

  <p>While there is no way to restore the consistency guarantee in
     an application transaction that consists of multiple database
     transactions, ODB provides a mechanism, called optimistic
     concurrency, that allows applications to detect and potentially
     recover from such inconsistencies.</p>

  <p>In essence, the optimistic concurrency model detects mismatches
     between the current object state in the database and the state
     when it was loaded into the application memory. Such a mismatch
     would mean that the object was changed by another process or
     thread. There are several ways to implement such state mismatch
     detection. Currently, ODB uses object versioning while other
     methods, such as timestamps, may be supported in the future.</p>

  <p>To declare a persistent class with the optimistic concurrency model we
     use the <code>optimistic</code> pragma (<a href="#14.1.5">Section 14.1.5,
     "<code>optimistic</code>"</a>). We also use the <code>version</code>
     pragma (<a href="#14.4.16">Section 14.4.16, "<code>version</code>"</a>)
     to specify which data member will store the object version. For
     example:</p>

  <pre class="cxx">
#pragma db object optimistic
class person
{
  ...

  #pragma db version
  unsigned long long version_;
};
  </pre>

  <p>The version data member is managed by ODB. It is initialized to
     <code>1</code> when the object is made persistent and incremented
     by <code>1</code> with each update. The <code>0</code> version value
     is not used by ODB and the application can use it as a special value,
     for example, to indicate that the object is transient. Note that
     for optimistic concurrency to function properly, the application
     should not modify the version member after making the object persistent
     or loading it from the database and until deleting the state of this
     object from the database. To avoid any accidental modifications
     to the version member, we can declare it <code>const</code>, for
     example:</p>

  <pre class="cxx">
#pragma db object optimistic
class person
{
  ...

  #pragma db version
  const unsigned long long version_;
};
  </pre>

  <p>When we call the <code>database::update()</code> function
     (<a href="#3.10">Section 3.10, "Updating Persistent Objects"</a>) and pass
     an object that has an outdated state, the <code>odb::object_changed</code>
     exception is thrown. At this point the application has two
     recovery options: it can abort and potentially restart the
     application transaction or it can reload the new object
     state from the database, re-apply or merge the changes, and call
     <code>update()</code> again. Note that aborting an application
     transaction that performs updates in multiple database transactions
     may require reverting changes that have already been committed to
     the database. As a result, this strategy works best if all the
     updates are performed in the last database transaction of the
     application transaction. This way the changes can be reverted
     by simply rolling back this last database transaction.</p>

  <p>The following example shows how we can reimplement the above
     transaction using the second recovery option:</p>

  <pre class="cxx">
unsigned long long id = ...;
person p;

{
  transaction t (db.begin ());
  db.load (id, p);
  t.commit ();
}

cerr &lt;&lt; "enter age for " &lt;&lt; p.first () &lt;&lt; " " &lt;&lt; p.last () &lt;&lt; endl;
unsigned short age;
cin >> age;
p.age (age);

{
  transaction t (db.begin ());

  try
  {
    db.update (p);
  }
  catch (const object_changed&amp;)
  {
    db.reload (p);
    p.age (age);
    db.update (p);
  }

  t.commit ();
}
  </pre>

  <p>An important point to note in the above code fragment is that the second
     <code>update()</code> call cannot throw the <code>object_changed</code>
     exception because we are reloading the state of the object
     and updating it within the same database transaction.</p>

  <p>Depending on the recovery strategy employed by the application,
     an application transaction with a failed update can be significantly
     more expensive than a successful one. As a result, optimistic
     concurrency works best for situations with low to medium contention
     levels where the majority of the application transactions complete
     without update conflicts. This is also the reason why this concurrency
     model is called optimistic.</p>

  <p>In addition to updates, ODB also performs state mismatch detection
     when we are deleting an object from the database
     (<a href="#3.11">Section 3.11, "Deleting Persistent Objects"</a>).
     To understand why this can be important, consider the following
     application transaction:</p>

  <pre class="cxx">
unsigned long long id = ...;
person p;

{
  transaction t (db.begin ());
  db.load (id, p);
  t.commit ();
}

string answer;
cerr &lt;&lt; "age is " &lt;&lt; p.age () &lt;&lt; ", delete?" &lt;&lt; endl;
getline (cin, answer);

if (answer == "yes")
{
  transaction t (db.begin ());
  db.erase (p);
  t.commit ();
}
  </pre>

  <p>Consider again what happens if another process or thread updates
     the object by changing the person's age while we are waiting for
     the user input. In this case, the user makes the decision based on
     a certain age while we may delete (or not delete) an object that has
     a completely different age. Here is how we can  fix this problem
     using optimistic concurrency:</p>

  <pre class="cxx">
unsigned long long id = ...;
person p;

{
  transaction t (db.begin ());
  db.load (id, p);
  t.commit ();
}

string answer;
for (bool done (false); !done; )
{
  if (answer.empty ())
    cerr &lt;&lt; "age is " &lt;&lt; p.age () &lt;&lt; ", delete?" &lt;&lt; endl;
  else
    cerr &lt;&lt; "age changed to " &lt;&lt; p.age () &lt;&lt; ", still delete?" &lt;&lt; endl;

  getline (cin, answer);

  if (answer == "yes")
  {
    transaction t (db.begin ());

    try
    {
      db.erase (p);
      done = true;
    }
    catch (const object_changed&amp;)
    {
      db.reload (p);
    }

    t.commit ();
  }
  else
    done = true;
}
  </pre>

  <p>Note that state mismatch detection is performed only if we delete
     an object by passing the object instance to the <code>erase()</code>
     function. If we want to delete an object with the optimistic concurrency
     model regardless of its state, then we need to use the <code>erase()</code>
     function that deletes an object given its id, for example:</p>

  <pre class="cxx">
{
  transaction t (db.begin ());
  db.erase (p.id ());
  t.commit ();
}
  </pre>

  <p>Finally, note that for persistent classes with the optimistic concurrency
     model both the <code>update()</code> function as well as the
     <code>erase()</code> function that accepts an object instance as its
     argument no longer throw the <code>object_not_persistent</code>
     exception if there is no such object in the database. Instead,
     this condition is treated as a change of object state and the
     <code>object_changed</code> exception is thrown instead.</p>

  <p>For complete sample code that shows how to use optimistic
     concurrency, refer to the <code>optimistic</code> example in
     the <code>odb-examples</code> package.</p>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="13">13 Database Schema Evolution</a></h1>

  <p>When we add new persistent classes or change the existing ones, for
     example, by adding or deleting data members, the database schema
     necessary to store the new object model changes as well. At the
     same time, we may have existing databases that contain existing data.
     If new versions of your application don't need to handle
     old databases, then the schema creating functionality is all that
     you need. However, most applications will need to work with data
     stored by older versions of the same application.</p>

  <p>We will call <em>database schema evolution</em> the overall task
     of updating the database to match the changes in the object model.
     Schema evolution usually consists of two sub-tasks: <em>schema
     migration</em> and <em>data migration</em>. Schema migration
     modifies the database schema to correspond to the current
     object model. In a relational database, this, for example, could
     require adding or dropping tables and columns. The data migration
     task involves converting the data stored in the existing database
     from the old format to the new one.</p>

  <p>If performed manually, database schema evolution is a tedious and
     error-prone task. As a result, ODB provides comprehensive support
     for automated or, more precisely, semi-automated schema
     evolution. Specifically, ODB does fully-automatic schema
     migration and provides facilities to help you with data
     migration.</p>

  <p>The topic of schema evolution is a complex and sensitive
     issue since normally there would be valuable, production data at
     stake. As a result, the approach taken by ODB is to provide simple
     and bullet-proof elementary building blocks (or migration steps)
     that we can understand and trust. Using these elementary blocks we
     can then implement more complex migration scenarios. In particular,
     ODB does not try to handle data migration automatically since in most
     cases this requires understanding of application-specific semantics.
     In other words, there is no magic.</p>

  <p>There are two general approaches to working with older data: the
     application can either convert it to correspond to the new format
     or it can be made capable of working with multiple versions of this
     format. There is also a hybrid approach where the application
     may convert the data to the new format gradually as part of its
     normal functionality. ODB is capable of handling all these
     scenarios. That is, there is support for working with older
     models without performing any migration (schema or data).
     Alternatively, we can migrate the schema after
     which we have the choice of either also immediately migrating the
     data (<em>immediate data migration</em>) or doing it gradually
     (<em>gradual data migration</em>).</p>

  <p>Schema evolution is already a complex task and we should not
     unnecessarily use a more complex approach where a simpler one
     would be sufficient. From the above, the simplest approach is
     the immediate schema migration that does not require any data
     migration. An example of such a change would be adding a new
     data member with the default value (<a href="#14.3.4">Section
     14.3.4, "<code>default</code>"</a>). This case ODB can handle
     completely automatically.</p>

  <p>If we do require data migration, then the next simplest approach
     is the immediate schema and data migration. Here we have to write
     custom migration code. However, it is separate from the rest of
     the core application logic and is executed at a well defined point
     (database migration). In other words, the core application logic
     need not be aware of older model versions. The potential drawback
     of this approach is performance. It may take a lot of resources
     and/or time to convert all the data upfront.</p>

  <p>If the immediate migration is not possible, then the next option
     is the immediate schema migration followed by the gradual data
     migration. With this approach, both old and new data must co-exist
     in the new database. We also have to change the application
     logic to both account for different sources of the same data (for
     example, when either an old or new version of the object is loaded)
     as well as migrate the data when appropriate (for example, when
     the old version of the object is updated). At some point, usually
     when the majority of the data has been converted, gradual migrations
     are terminated with an immediate migration.</p>

  <p>The most complex approach is working with multiple versions of
     the database without performing any migrations, schema or data.
     ODB does provide support for implementing this approach
     (<a href="#13.4">Section 13.4, "Soft Object Model Changes"</a>),
     however we will not cover it any further in this chapter.
     Generally, this will require embedding knowledge about each
     version into the core application logic which makes it hard
     to maintain for any non-trivial object model.</p>

  <p>Note also that when it comes to data migration, we can use
     the immediate variant for some changes and gradual for others.
     We will discuss various migration scenarios in greater detail
     in section <a href="#13.3">Section 13.3, "Data Migration"</a>.</p>

  <h2><a name="13.1">13.1 Object Model Version and Changelog</a></h2>

  <p>To enable schema evolution support in ODB we need to specify
     the object model version, or, more precisely, two versions.
     The first is the base model version. It is the lowest
     version from which we will be able to migrate. The second
     version is the current model version. In ODB we can migrate
     from multiple previous versions by successively migrating
     from one to the next until we reach the current version.
     We use the <code>db&nbsp;model&nbsp;version</code> pragma
     to specify both the base and current versions.</p>

  <p>When we enable schema evolution for the first time, our
     base and current versions will be the same, for example:</p>

  <pre class="cxx">
#pragma db model version(1, 1)
  </pre>

  <p>Once we release our application, its users may create databases
     with the schema corresponding to this version of the object
     model. This means that if we make any modifications to our
     object model that also change the schema, then we will need
     to be able to migrate the old databases to this new schema.
     As a result, before making any new changes after a release,
     we increment the current version, for example:</p>

  <pre class="cxx">
#pragma db model version(1, 2)
  </pre>

  <p>To put this another way, we can stay on the same version
     during development and keep adding new changes to it. But
     once we release it, any new changes to the object model will
     have to be done in a new version.</p>

  <p>It is easy to forget to increment the version before
     making new changes to the object model. To help solve this
     problem, the <code>db&nbsp;model&nbsp;version</code> pragma
     accepts a third optional argument that specify whether the
     current version is open or closed for changes. For example:</p>

  <pre class="cxx">
#pragma db model version(1, 2, open)   // Can add new changes to
                                       // version 2.
  </pre>

  <pre class="cxx">
#pragma db model version(1, 2, closed) // Can no longer add new
                                       // changes to version 2.
  </pre>

  <p>If the current version is closed, ODB will refuse to accept
     any new schema changes. In this situation you would
     normally increment the current version and mark it as open
     or you could re-open the existing version if, for example,
     you need to fix something. Note, however, that re-opening
     versions that have been released will most likely result
     in migration malfunctions. By default the version is open.</p>

  <p>Normally, an application will have a range of older database
     versions from which it is able to migrate. When we change
     this range by removing support for older versions, we also
     need to adjust the base model version. This will make sure
     that ODB does not keep unnecessary information around.</p>

  <p>A model version (both base and current) is a 64-bit unsigned
     integer (<code>unsigned&nbsp;long&nbsp;long</code>). <code>0</code>
     is reserved to signify special situations, such as the lack of
     schema in the database. Other than that, we can use any values
     as versions as long as they are monotonically increasing. In
     particular, we don't have to start with version <code>1</code>
     and can increase the versions by any increment.</p>

  <p>One versioning approach is to use an independent
     object model version by starting from version <code>1</code>
     and also incrementing by <code>1</code>. The alternative
     is to make the model version correspond to the application
     version. For example, if our application is using the
     <code>X.Y.Z</code> version format, then we could encode it
     as a hexadecimal number and use that as our model version,
     for example:</p>

  <pre class="cxx">
#pragma db model version(0x020000, 0x020306) // 2.0.0-2.3.6
  </pre>

  <p>Most real-world object models will be spread over multiple
     header files and it will be burdensome to repeat the
     <code>db&nbsp;model&nbsp;version</code> pragma in each of
     them. The recommended way to handle this situation is to
     place the <code>version</code> pragma into a separate header
     file and include it into the object model files. If your
     project already has a header file that defines the
     application version, then it is natural to place this
     pragma there. For example:</p>

  <pre class="cxx">
// version.hxx
//
// Define the application version.
//

#define MYAPP_VERSION 0x020306 // 2.3.6

#ifdef ODB_COMPILER
#pragma db model version(1, 7)
#endif
  </pre>

  <p>Note that we can also use macros in the <code>version</code>
     pragma which allows us to specify all the versions in a single
     place. For example:</p>

  <pre class="cxx">
#define MYAPP_VERSION      0x020306 // 2.3.6
#define MYAPP_BASE_VERSION 0x020000 // 2.0.0

#ifdef ODB_COMPILER
#pragma db model version(MYAPP_BASE_VERSION, MYAPP_VERSION)
#endif
  </pre>

  <p>It is also possible to have multiple object models within the
     same application that have different versions. Such models
     must be independent, that is, no headers from one model shall
     include a header from another. You will also need to assign
     different schema names to each model with the
     <code>--schema-name</code> ODB compiler option.</p>

  <p>Once we specify the object model version, the ODB compiler
     starts tracking database schema changes in a changelog file.
     Changelog has an XML-based, line-oriented format. It uses
     XML in order to provide human readability while also
     facilitating, if desired, processing and analysis with
     custom tools. The line orientation makes it easy to review
     with tools like <code>diff</code>.</p>

  <p>The changelog is maintained by the ODB compiler. Specifically,
     you do not need to make any manual changes to this file. You
     will, however, need to keep it around from one invocation of
     the ODB compiler to the next. In other words, the changelog
     file is both the input and the output of the ODB compiler. This,
     for example, means that if your project's source code is stored
     in a version control repository, then you will most likely want
     to store the changelog there as well. If you delete the changelog,
     then any ability to do schema migration will be lost.</p>

  <p>The only operation that you may want to perform with the
     changelog is to review the database schema changes that resulted
     from the C++ object model changes. For this you can use a tool
     like <code>diff</code> or, better yet, the change review facilities
     offered by your revision control system. For this purpose the
     contents of a changelog will be self-explanatory.</p>

  <p>As an example, consider the following initial object model:</p>

  <pre class="cxx">
// person.hxx
//

#include &lt;string>

#pragma db model version(1, 1)

#pragma db object
class person
{
  ...

  #pragma db id auto
  unsigned long long id_;

  std::string first_;
  std::string last_;
};
  </pre>

  <p>We then compile this header file with the ODB compiler (using the
     PostgreSQL database as an example):</p>

  <pre class="terminal">
odb --database pgsql --generate-schema person.hxx
  </pre>

  <p>If we now look at the list of generated files, then in addition to
     the now familiar <code>person-odb.?xx</code> and <code>person.sql</code>,
     we will also see <code>person.xml</code> &mdash; the changelog file.
     Just for illustration, below are the contents of this changelog.</p>

  <pre class="xml">
&lt;changelog database="pgsql">
  &lt;model version="1">
    &lt;table name="person" kind="object">
      &lt;column name="id" type="BIGINT" null="false"/>
      &lt;column name="first" type="TEXT" null="false"/>
      &lt;column name="last" type="TEXT" null="false"/>
      &lt;primary-key auto="true">
        &lt;column name="id"/>
      &lt;/primary-key>
    &lt;/table>
  &lt;/model>
&lt;/changelog>
  </pre>

  <p>Let's say we now would like to add another data member to the
     <code>person</code> class &mdash; the middle name. We increment
     the version and make the change:</p>

  <pre class="cxx">
#pragma db model version(1, 2)

#pragma db object
class person
{
  ...

  #pragma db id auto
  unsigned long long id_;

  std::string first_;
  std::string middle_;
  std::string last_;
};
  </pre>

  <p>We use exactly the same command line to re-compile our file:</p>

  <pre class="terminal">
odb --database pgsql --generate-schema person.hxx
  </pre>

  <p>This time the ODB compiler will read the old changelog, update
     it, and write out the new version. Again, for illustration only,
     below are the updated changelog contents:</p>

  <pre class="xml">
&lt;changelog database="pgsql">
  &lt;changeset version="2">
    &lt;alter-table name="person">
      &lt;add-column name="middle" type="TEXT" null="false"/>
    &lt;/alter-table>
  &lt;/changeset>

  &lt;model version="1">
    &lt;table name="person" kind="object">
      &lt;column name="id" type="BIGINT" null="false"/>
      &lt;column name="first" type="TEXT" null="false"/>
      &lt;column name="last" type="TEXT" null="false"/>
      &lt;primary-key auto="true">
        &lt;column name="id"/>
      &lt;/primary-key>
    &lt;/table>
  &lt;/model>
&lt;/changelog>
  </pre>

  <p>Just to reiterate, while the changelog may look like it could
     be written by hand, it is maintained completely automatically
     by the ODB compiler and the only reason you may want to look
     at its contents is to review the database schema changes. For
     example, if we compare the above two changelogs with
     <code>diff</code>, we will get the following summary of the
     database schema changes:</p>

  <pre class="xml">
--- person.xml.orig
+++ person.xml
@@ -1,4 +1,10 @@
&lt;changelog database="pgsql">
<span style="color: #009E00">+  &lt;changeset version="2">
+    &lt;alter-table name="person">
+      &lt;add-column name="middle" type="TEXT" null="false"/>
+    &lt;/alter-table>
+  &lt;/changeset>
+</span>
  &lt;model version="1">
    &lt;table name="person" kind="object">
      &lt;column name="id" type="BIGINT" null="false"/>
  </pre>

  <p>The changelog is only written when we generate the database schema,
     that is, the <code>--generate-schema</code> option is specified.
     Invocations of the ODB compiler that only produce the database
     support code (C++) do not read or update the changelog. To put it
     another way, the changelog tracks changes in the resulting database
     schema, not the C++ object model.</p>

  <p>ODB ignores column order when comparing database schemas. This means
     that we can re-order data members in a class without causing any
     schema changes. Member renames, however, will result in schema
     changes since the column name changes as well (unless we specified
     the column name explicitly). From ODB's perspective such a rename
     looks like the deletion of one data member and the addition of
     another. If we don't want this to be treated as a schema change,
     then we will need to keep the old column name by explicitly
     specifying it with the <code>db&nbsp;column</code> pragma. For
     example, here is how we can rename <code>middle_</code> to
     <code>middle_name_</code> without causing any schema changes:</p>

  <pre class="cxx">
#pragma db model version(1, 2)

#pragma db object
class person
{
  ...

  #pragma db column("middle") // Keep the original column name.
  std::string middle_name_;

  ...
};
  </pre>

  <p>If your object model consists of a large number of header files and
     you generate the database schema for each of them individually, then
     a changelog will be created for each of your header files. This may
     be what you want, however, the large number of changelogs can quickly
     become unwieldy. In fact, if you are generating the database schema
     as standalone SQL files, then you may have already experienced a
     similar problem caused by a large number of <code>.sql</code> files,
     one for each header.</p>

  <p>The solution to both of these problems is to generate a combined
     database schema file and a single changelog. For example, assume
     we have three header files in our object model:
     <code>person.hxx</code>, <code>employee.hxx</code>, and
     <code>employer.hxx</code>. To generate the database support code
     we compile them as usual but without specifying the
     <code>--generate-schema</code> option. In this case no changelog
     is created or updated:</p>

  <pre class="terminal">
odb --database pgsql person.hxx
odb --database pgsql employee.hxx
odb --database pgsql employer.hxx
  </pre>

  <p>To generate the database schema, we perform a separate invocation
     of the ODB compiler. This time, however, we instruct it to only
     generate the schema (<code>--generate-schema-only</code>) and
     produce it combined (<code>--at-once</code>) for all the files
     in our object model:</p>

  <pre class="terminal">
odb --database pgsql --generate-schema-only --at-once \
--input-name company person.hxx employee.hxx employer.hxx
  </pre>

  <p>The result of the above command is a single <code>company.sql</code>
     file (the name is derived from the <code>--input-name</code> value)
     that contains the database schema for our entire object model. There
     is also a single corresponding changelog file &mdash;
     <code>company.xml</code>.</p>

  <p>The same can be achieved for the embedded schema by instructing
     the ODB compiler to generate the database creation code into a
     separate C++ file (<code>--schema-format&nbsp;separate</code>):</p>

  <pre class="terminal">
odb --database pgsql --generate-schema-only --schema-format separate \
--at-once --input-name company person.hxx employee.hxx employer.hxx
  </pre>

  <p>The result of this command is a single <code>company-schema.cxx</code>
     file and, again, <code>company.xml</code>.</p>

  <p>Note also that by default the changelog file is not placed into
     the directory specified with the <code>--output-dir</code> option.
     This is due to the changelog being both an input and an output file
     at the same time. As a result, by default, the ODB compiler will
     place it in the directory of the input header file.</p>

  <p>There is, however, a number of command line options (including
     <code>--changelog-dir</code>) that allow us to fine-tune the name and
     location of the changelog file. For example, you can instruct the ODB
     compiler to read the changelog from one file while writing it to
     another. This, for example, can be useful if you want to review
     the changes before discarding the old file. For more information
     on these options, refer to the
     <a href="http://www.codesynthesis.com/products/odb/doc/odb.xhtml">ODB
     Compiler Command Line Manual</a> and search for "changelog".</p>

  <p>When we were discussing version increments above, we used the
     terms <em>development</em> and <em>release</em>. Specifically,
     we talked about keeping the same object model versions during
     development periods and incrementing them after releases.
     What is a development period and a release in this context?
     These definitions can vary from project to project.
     Generally, during a development period we work on one or
     more changes to the object model that result in the changes
     to the database schema. A release is a point where we
     make our changes available to someone else who may have an
     older database to migrate from. In the traditional sense, a release
     is a point where you make a new version of your application available
     to its users. However, for schema evolution purposes, a release
     could also mean simply making your schema-altering changes
     available to other developers on your team. Let us consider
     two common scenarios to illustrate how all this fits together.</p>

  <p>One way to setup a project would be to re-use the application
     development period and application release for schema evolution.
     That is, during a new application version development we keep
     a single object model version and when we release the application,
     we increment the model version. In this case it makes sense to
     also reuse the application version as a model version for
     consistency. Here is a step-by-step guide for this setup:</p>

  <ol>
    <li>During development, keep the current object model version open.</li>

    <li>Before the release (for example, when entering a "feature freeze")
        close the version.</li>

    <li>After the release, update the version and open it.</li>

    <li>For each new feature, review the changeset at the top of the
        changelog, for example, with <code>diff</code> or your
        version control facilities. If you are using a version
        control, then this is best done just before committing
        your changes to the repository.</li>
  </ol>

  <p>An alternative way to setup schema versioning in a project would
     be to define the development period as working on a single
     feature and the release as making this feature available to
     other people (developers, testers, etc.) on your team, for
     example, by committing the changes to a public version control
     repository. In this case, the object model version will be
     independent of the application version and can simply be
     a sequence that starts with <code>1</code> and is
     incremented by <code>1</code>. Here is a step-by-step guide
     for this setup:</p>

  <ol>
    <li>Keep the current model version closed. Once a change is made
        that affects the database schema, the ODB compiler will refuse
        to update the changelog.</li>

    <li>If the change is legitimate, open a new version, that is,
        increment the current version and make it open.</li>

    <li>Once the feature is implemented and tested, review the final
        set of database changes (with <code>diff</code> or your
        version control facilities), close the version, and commit
        the changes to the version control repository (if using).</li>
  </ol>

  <p>If you are using a version control repository that supports
     pre-commit checks, then you may want to consider adding such
     a check to make sure the committed version is always closed.</p>

  <p>If we are just starting schema evolution in our project, which
     approach should we choose? The two approaches will work better
     in different situations since they have a different set of
     advantages and disadvantages. The first approach, which we
     can call version per application release, is best suited
     for simpler projects with smaller releases since otherwise
     a single migration will bundle a large number of unrelated
     actions corresponding to different features. This can
     become difficult to review and, if things go wrong, debug.</p>

  <p>The second approach, which we can call version per feature,
     is much more modular and provides a number of additional benefits.
     We can perform migrations for each feature as a discreet step
     which makes it easier to debug. We can also place each such
     migration step into a separate transaction further improving
     reliability. It also scales much better in larger teams
     where multiple developers can work concurrently on features
     that affect the database schema. For example, if you find
     yourself in a situation where another developer on your
     team used the same version as you and managed to commit his
     changes before you (that is, you have a merge conflict),
     then you can simply change the version to the next available
     one, regenerate the changelog, and continue with your commit.</p>

  <p>Overall, unless you have strong reasons to prefer the version
     per application release approach, rather choose version per
     feature even though it may seem more complex at the
     beginning. Also, if you do select the first approach, consider
     provisioning for switching to the second method by reserving
     a sub-version number. For example, for an application version
     in the form <code>2.3.4</code> you can make the object model
     version to be in the form <code>0x0203040000</code>, reserving
     the last two bytes for a sub-version. Later on you can use it to
     switch to the version per feature approach.</p>

  <h2><a name="13.2">13.2 Schema Migration</a></h2>

  <p>Once we enable schema evolution by specifying the object model
     version, in addition to the schema creation statements, the
     ODB compiler starts generating schema migration statements
     for each version all the way from the base to the current.
     As with schema creation, schema migration can be generated
     either as a set of SQL files or embedded into the generated
     C++ code (<code>--schema-format</code> option).</p>

  <p>For each migration step, that is from one version to the next,
     ODB generates two sets of statements: pre-migration and
     post-migration. The pre-migration statements <em>"relax"</em>
     the database schema so that both old and new data can co-exist.
     At this stage new columns and tables are added while old
     constraints are dropped. The post-migration statements
     <em>"tighten"</em> the database schema back so that only
     data conforming to the new format can remain. At this stage
     old columns and tables are dropped and new constraints are
     added. Now you can probably guess where the data
     migration fits into this &mdash; between the pre and post
     schema migrations where we can both access the old data
     and create the new one.</p>

  <p>If the schema is being generated as standalone SQL files,
     then we end up with a pair of files for each step: the pre-migration
     file and the post-migration file. For the <code>person</code>
     example we started in the previous section we will have the
     <code>person-002-pre.sql</code> and <code>person-002-post.sql</code>
     files. Here <code>002</code> is the version <em>to</em> which
     we are migrating while the <code>pre</code> and <code>post</code>
     suffixes specify the migration stage. So if we wanted to migrate
     a <code>person</code> database from version <code>1</code>
     to <code>2</code>, then we would first execute
     <code>person-002-pre.sql</code>, then migrate the data, if any
     (discussed in more detail in the next section), and finally
     execute <code>person-002-post.sql</code>. If our database is
     several versions behind, for example the database has version
     <code>1</code> while the current version is <code>5</code>,
     then we simply perform this set of steps for each version
     until we reach the current version.</p>

  <p>If we look at the contents of the <code>person-002-pre.sql</code>
     file, we will see the following (or equivalent, depending on the
     database used) statement:</p>

  <pre class="sql">
ALTER TABLE "person"
  ADD COLUMN "middle" TEXT NULL;
  </pre>

  <p>As we would expect, this statement adds a new column corresponding
     to the new data member. An observant reader would notice,
     however, that the column is added as <code>NULL</code>
     even though we never requested this semantics in our object model.
     Why is the column added as <code>NULL</code>? If during migration
     the <code>person</code> table already contains rows (that is, existing
     objects), then an attempt to add a non-<code>NULL</code> column that
     doesn't have a default value will fail. As a result, ODB will initially
     add a new column that doesn't have a default value as <code>NULL</code>
     but then clean this up at the post-migration stage. This way your data
     migration code is given a chance to assign some meaningful values for
     the new data member for all the existing objects. Here are the contents
     of the <code>person-002-post.sql</code> file:</p>

  <pre class="sql">
ALTER TABLE "person"
  ALTER COLUMN "middle" SET NOT NULL;
  </pre>

  <p>Currently ODB directly supports the following elementary database
     schema changes:</p>

  <ul class="list">
    <li>add table</li>
    <li>drop table</li>
    <li>add column</li>
    <li>drop column</li>
    <li>alter column, set <code>NULL</code>/<code>NOT NULL</code></li>
    <li>add foreign key</li>
    <li>drop foreign key</li>
    <li>add index</li>
    <li>drop index</li>
  </ul>

  <p>More complex changes can normally be implemented in terms of
     these building blocks. For example, to change a type of a
     data member (which leads to a change of a column type), we
     can add a new data member with the desired type (add column),
     migrate the data, and then delete the old data member (drop
     column). ODB will issue diagnostics for cases that are
     currently not supported directly. Note also that some database
     systems (notably SQLite) have a number of limitations in their
     support for schema changes. For more information on these
     database-specific limitations, refer to the "Limitations" sections
     in <a href="#II">Part II, "Database Systems"</a>.</p>

  <p>How do we know what the current database version is? That is, the
     version <em>from</em> which we need to migrate? We need to know this,
     for example, in order to determine the set of migrations we have to
     perform. By default, when schema evolution is enabled, ODB maintains
     this information in a special table called <code>schema_version</code>
     that has the following (or equivalent, depending on the database
     used) definition:</p>

  <pre class="sql">
CREATE TABLE "schema_version" (
  "name" TEXT NOT NULL PRIMARY KEY,
  "version" BIGINT NOT NULL,
  "migration" BOOLEAN NOT NULL);
  </pre>

  <p>The <code>name</code> column is the schema name as specified with
     the <code>--schema-name</code> option. It is empty for the default
     schema. The <code>version</code> column contains the current database
     version. And, finally, the <code>migration</code> flag indicates
     whether we are in the process of migrating the database, that is,
     between the pre and post-migration stages.</p>

  <p>The schema creation statements (<code>person.sql</code> in our case)
     create this table and populate it with the initial model version. For
     example, if we executed <code>person.sql</code> corresponding to
     version <code>1</code> of our object model, then <code>name</code>
     would have been empty (which signifies the default schema since we
     didn't specify <code>--schema-name</code>), <code>version</code> will
     be <code>1</code> and <code>migration</code> will be
     <code>FALSE</code>.</p>

  <p>The pre-migration statements update the version and set the migration
     flag to <code>TRUE</code>. Continuing with our example, after executing
     <code>person-002-pre.sql</code>, <code>version</code> will
     become <code>2</code> and <code>migration</code> will be set to
     <code>TRUE</code>. The post-migration statements simply clear the
     migration flag. In our case, after running
     <code>person-002-post.sql</code>, <code>version</code> will
     remain <code>2</code> while <code>migration</code> will be reset
     to <code>FALSE</code>.</p>

  <p>Note also that above we mentioned that the schema creation statements
     (<code>person.sql</code>) create the <code>schema_version</code> table.
     This means that if we enable schema evolution support in the middle
     of a project, then we could already have existing databases that
     don't include this table. As a result, ODB will not be able to handle
     migrations for such databases unless we manually add the
     <code>schema_version</code> table and populate it with the correct
     version information. For this reason, it is highly recommended that
     you consider whether to use schema evolution and, if so, enable it
     from the beginning of your project.</p>

  <p>The <code>odb::database</code> class provides an API for accessing
     and modifying the current database version:</p>

  <pre class="cxx">
namespace odb
{
  using schema_version = unsigned long long;

  struct LIBODB_EXPORT schema_version_migration
  {
    schema_version_migration (schema_version = 0,
                              bool migration = false);

    schema_version version;
    bool migration;

    // This class also provides the ==, !=, &lt;, >, &lt;=, and >= operators.
    // Version ordering is as follows: {1,f} &lt; {2,t} &lt; {2,f} &lt; {3,t}.
  };

  class database
  {
  public:
    ...

    schema_version
    schema_version (const std::string&amp; name = "") const;

    bool
    schema_migration (const std::string&amp; name = "") const;

    const schema_version_migration&amp;
    schema_version_migration (const std::string&amp; name = "") const;

    // Set schema version and migration state manually.
    //
    void
    schema_version_migration (schema_version,
                              bool migration,
                              const std::string&amp; name = "");

    void
    schema_version_migration (const schema_version_migration&amp;,
                              const std::string&amp; name = "");

    // Set default schema version table for all schemas.
    //
    void
    schema_version_table (const std::string&amp; table_name);

    // Set schema version table for a specific schema.
    //
    void
    schema_version_table (const std::string&amp; table_name,
                          const std::string&amp; name);
  };
}
  </pre>

  <p>The <code>schema_version()</code> and <code>schema_migration()</code>
     accessors return the current database version and migration flag,
     respectively. The optional <code>name</code> argument is the schema
     name. If the database schema hasn't been created (that is, there is
     no corresponding entry in the <code>schema_version</code> table or
     this table does not exist), then <code>schema_version()</code> returns
     <code>0</code>. The <code>schema_version_migration()</code> accessor
     returns both version and migration flag together in the
     <code>schema_version_migration</code> <code>struct</code>.</p>

  <p>You may already have a version table in your database or you (or your
     database administrator) may prefer to keep track of versions your own
     way. You can instruct ODB not to create the <code>schema_version</code>
     table with the <code>--suppress-schema-version</code> option. However,
     ODB still needs to know the current database version in order for certain
     schema evolution mechanisms to function properly. As a result, in
     this case, you will need to set the schema version on the database
     instance manually using the schema_version_migration() modifier.
     Note that the modifier API is not thread-safe. That is, you should
     not modify the schema version while other threads may be accessing
     or modifying the same information.</p>

  <p>Note also that the accessors we discussed above will only query the
     <code>schema_version</code> table once and, if the version could
     be determined, cache the result. If, however, the version could
     not be determined (that is, <code>schema_version()</code> returned
     0), then a subsequent call will re-query the table. While it is
     probably a bad idea to modify the database schema while the
     application is running (other than via the <code>schema_catalog</code>
     API, as discussed below), if for some reason you need ODB to re-query
     the version, then you can manually set it to 0 using the
     <code>schema_version_migration()</code> modifier.</p>

  <p>It is also possible to change the name of the table that stores
     the schema version using the <code>--schema-version-table</code>
     option. You will also need to specify this alternative name on
     the <code>database</code> instance using the <code>schema_version_table()</code>
     modifier. The first version specifies the default table that is
     used for all the schema names. The second version specifies the
     table for a specific schema. The table name should be
     database-quoted, if necessary.</p>

  <p>If we are generating our schema migrations as standalone SQL files,
     then the migration workflow could look like this:</p>

  <ol>
    <li>The database administrator determines the current database version.
        If migration is required, then for each migration step (that
        is, from one version to the next), they perform the following:</li>

    <li>Execute the pre-migration file.</li>

    <li>Execute our application (or a separate migration program)
        to perform data migration (discussed later). Our application
        can determine that is is being executed in the "migration mode"
        by calling <code>schema_migration()</code> and then which
        migration code to run by calling <code>schema_version()</code>.</li>

    <li>Execute the post-migration file.</li>
  </ol>

  <p>These steps become more integrated and automatic if we embed the
     schema creation and migration code into the generated C++ code.
     Now we can perform schema creation, schema migration, and data
     migration as well as determine when each step is necessary
     programmatically from within the application.</p>

  <p>Schema evolution support adds the following extra functions to
     the <code>odb::schema_catalog</code> class, which we first discussed
     in <a href="#3.4">Section 3.4, "Database"</a>.</p>

  <pre class="cxx">
namespace odb
{
  class schema_catalog
  {
  public:
    ...


    // Schema migration.
    //
    static void
    migrate_schema_pre (database&amp;,
                        schema_version,
                        const std::string&amp; name = "");

    static void
    migrate_schema_post (database&amp;,
                         schema_version,
                         const std::string&amp; name = "");

    static void
    migrate_schema (database&amp;,
                    schema_version,
                    const std::string&amp; name = "");

    // Data migration.
    //
    // Discussed in the next section.


    // Combined schema and data migration.
    //
    static void
    migrate (database&amp;,
             schema_version = 0,
             const std::string&amp; name = "");

    // Schema version information.
    //
    static schema_version
    base_version (const database&amp;,
                  const std::string&amp; name = "");

    static schema_version
    base_version (database_id,
                  const std::string&amp; name = "");

    static schema_version
    current_version (const database&amp;,
                     const std::string&amp; name = "");

    static schema_version
    current_version (database_id,
                     const std::string&amp; name = "");

    static schema_version
    next_version (const database&amp;,
                  schema_version = 0,
                  const std::string&amp; name = "");

    static schema_version
    next_version (database_id,
                  schema_version,
                  const std::string&amp; name = "");
  };
}
  </pre>

  <p>The <code>migrate_schema_pre()</code> and
     <code>migrate_schema_post()</code> static functions perform
     a single stage (that is, pre or post) of a single migration
     step (that is, from one version to the next). The <code>version</code>
     argument specifies the version we are migrating to. For
     instance, in our <code>person</code> example, if we know that
     the database version is <code>1</code> and the next version
     is <code>2</code>, then we can execute code like this:</p>

  <pre class="cxx">
transaction t (db.begin ());

schema_catalog::migrate_schema_pre (db, 2);

// Data migration goes here.

schema_catalog::migrate_schema_post (db, 2);

t.commit ();
  </pre>

  <p>If you don't have any data migration code to run, then you can
     perform both stages with a single call using the
     <code>migrate_schema()</code> static function.</p>

  <p>The <code>migrate()</code> static function perform both schema
     and data migration (we discuss data migration in the next section).
     It can also perform several migration steps at once. If we don't
     specify its target version, then it will migrate (if necessary)
     all the way to the current model version. As an extra convenience,
     <code>migrate()</code> will also create the database schema if
     none exists. As a result, if we don't have any data migration
     code or we have registered it with <code>schema_catalog</code> (as
     discussed later), then the database schema creation and migration,
     whichever is necessary, if at all, can be performed with a single
     function call:</p>

  <pre class="cxx">
transaction t (db.begin ());
schema_catalog::migrate (db);
t.commit ();
  </pre>

  <p>Note also that <code>schema_catalog</code> is integrated with the
    <code>odb::database</code> schema version API. In particular,
    <code>schema_catalog</code> functions will query and synchronize
    the schema version on the <code>database</code> instance if and
    when required.</p>

  <p>The <code>schema_catalog</code> class also allows you to iterate
     over known versions (remember, there could be "gaps" in version
     numbers) with the <code>base_version()</code>,
     <code>current_version()</code> and <code>next_version()</code>
     static functions. The <code>base_version()</code> and
     <code>current_version()</code> functions return the base and
     current object model versions, respectively. That is, the
     lowest version from which we can migrate and the version that
     we ultimately want to migrate to. The <code>next_version()</code>
     function returns the next known version. If the passed version is
     greater or equal to the current version, then this function
     will return the current version plus one (that is, one past
     current). If we don't specify the version, then
     <code>next_version()</code> will use the current database version
     as the starting point. Note also that the schema version information
     provided by these functions is only available if we embed the schema
     migration code into the generated C++ code. For standalone SQL file
     migrations this information is normally not needed since the migration
     process is directed by an external entity, such as a database
     administrator or a script.</p>

  <p>Most <code>schema_catalog</code> functions presented above also accept
     the optional schema name argument. If the passed schema name is not
     found, then the <code>odb::unknown_schema</code> exception is
     thrown. Similarly, functions that accept the schema version argument will
     throw the <code>odb::unknown_schema_version</code> exception if the
     passed or current version is invalid. Refer to <a href="#3.14">Section
     3.14, "ODB Exceptions"</a> for more information on these exceptions.</p>

  <p>To illustrate how all these parts fit together, consider the
     following more realistic database schema management example.
     Here we want to handle the schema creation in a special way
     and perform each migration step in its own transaction.</p>

  <pre class="cxx">
schema_version v (db.schema_version ());
schema_version bv (schema_catalog::base_version (db));
schema_version cv (schema_catalog::current_version (db));

if (v == 0)
{
  // No schema in the database. Create the schema and
  // initialize the database.
  //
  transaction t (db.begin ());
  schema_catalog::create_schema (db);

  // Populate the database with initial data, if any.

  t.commit ();
}
else if (v &lt; cv)
{
  // Old schema (and data) in the database, migrate them.
  //

  if (v &lt; bv)
  {
    // Error: migration from this version is no longer supported.
  }

  for (v = schema_catalog::next_version (db, v);
       v &lt;= cv;
       v = schema_catalog::next_version (db, v))
  {
    transaction t (db.begin ());
    schema_catalog::migrate_schema_pre (db, v);

    // Data migration goes here.

    schema_catalog::migrate_schema_post (db, v);
    t.commit ();
  }
}
else if (v > cv)
{
  // Error: old application trying to access new database.
}
  </pre>

  <h2><a name="13.3">13.3 Data Migration</a></h2>

  <p>In quite a few cases specifying the default value for new data
     members will be all that's required to handle the existing objects.
     For example, the natural default value for the new middle name
     that we have added is an empty string. And we can handle
     this case with the <code>db&nbsp;default</code> pragma and without
     any extra C++ code:</p>

  <pre class="cxx">
#pragma db model version(1, 2)

#pragma db object
class person
{
  ...


  #pragma db default("")
  std::string middle_;
};
  </pre>

  <p>However, there will be situations where we would need to perform
     more elaborate data migrations, that is, convert old data to the
     new format. As an example, suppose we want to add gender to our
     <code>person</code> class. And, instead of leaving it unassigned
     for all the existing objects, we will try to guess it from the
     first name. This is not particularly accurate but it could be
     sufficient for our hypothetical application:</p>

  <pre class="cxx">
#pragma db model version(1, 3)

enum gender {male, female};

#pragma db object
class person
{
  ...

  gender gender_;
};
  </pre>

  <p>As we have discussed earlier, there are two ways to perform data
     migration: immediate and gradual. To recap, with immediate
     migration we migrate all the existing objects at once, normally
     after the schema pre-migration statements but before the
     post-migration statements. With gradual migration, we make sure
     the new object model can accommodate both old and new data and
     gradually migrate existing objects as the application runs and
     the opportunities to do so arise, for example, an object is
     updated.</p>

  <p>There is also another option for data migration that is not
     discussed further in this section. Instead of using our C++
     object model we could execute ad-hoc SQL statements that
     perform the necessary conversions and migrations directly
     on the database server. While in certain cases this can be
     a better option from the performance point of view, this
     approach is often limited in terms of the migration logic
     that we can handle.</p>

  <h2><a name="13.3.1">13.3.1 Immediate Data Migration</a></h2>

  <p>Let's first see how we can implement an immediate migration for the
     new <code>gender_</code> data member we have added above. If we
     are using standalone SQL files for migration, then we could add
     code along these lines somewhere early in <code>main()</code>,
     before the main application logic:</p>

  <pre class="cxx">
int
main ()
{
  ...

  odb::database&amp; db = ...

  // Migrate data if necessary.
  //
  if (db.schema_migration ())
  {
    switch (db.schema_version ())
    {
    case 3:
      {
        // Assign gender to all the existing objects.
        //
        transaction t (db.begin ());

        for (person&amp; p: db.query&lt;person> ())
        {
          p.gender (guess_gender (p.first ()));
          db.update (p);
        }

        t.commit ();
        break;
      }
    }
  }

  ...
}
  </pre>

  <p>If you have a large number of objects to migrate, it may also be
     a good idea, from the performance point of view, to break one big
     transaction that we now have into multiple smaller transactions
     (<a href="#3.5">Section 3.5, "Transactions"</a>). For example:</p>

  <pre class="cxx">
case 3:
  {
    transaction t (db.begin ());

    size_t n (0);
    for (person&amp; p: db.query&lt;person> ())
    {
      p.gender (guess_gender (p.first ()));
      db.update (p);

      // Commit the current transaction and start a new one after
      // every 100 updates.
      //
      if (n++ % 100 == 0)
      {
        t.commit ();
        t.reset (db.begin ());
      }
    }

    t.commit ();
    break;
  }
  </pre>

  <p>While it looks straightforward enough, as we add more migration
     snippets, this approach can quickly become unmaintainable. Instead
     of having all the migrations in a single function and determining
     when to run each piece ourselves, we can package each migration into
     a separate function, register it with the <code>schema_catalog</code>
     class, and let ODB figure out when to run which migration functions.
     To support this functionality, <code>schema_catalog</code> provides
     the following data migration API:</p>

  <pre class="cxx">
namespace odb
{
  class schema_catalog
  {
  public:
    ...

    // Data migration.
    //
    static std::size_t
    migrate_data (database&amp;,
                  schema_version = 0,
                  const std::string&amp; name = "");

    using data_migration_function_type = void (database&amp;);

    // Common (for all the databases) data migration, C++98/03 version:
    //
    template &lt;schema_version v, schema_version base>
    static void
    data_migration_function (data_migration_function_type*,
                             const std::string&amp; name = "");

    // Common (for all the databases) data migration, C++11 version:
    //
    template &lt;schema_version v, schema_version base>
    static void
    data_migration_function (std::function&lt;data_migration_function_type>,
                             const std::string&amp; name = "");

    // Database-specific data migration, C++98/03 version:
    //
    template &lt;schema_version v, schema_version base>
    static void
    data_migration_function (database&amp;,
                             data_migration_function_type*,
                             const std::string&amp; name = "");

    template &lt;schema_version v, schema_version base>
    static void
    data_migration_function (database_id,
                             data_migration_function_type*,
                             const std::string&amp; name = "");

    // Database-specific data migration, C++11 version:
    //
    template &lt;schema_version v, schema_version base>
    static void
    data_migration_function (database&amp;,
                             std::function&lt;data_migration_function_type>,
                             const std::string&amp; name = "");

    template &lt;schema_version v, schema_version base>
    static void
    data_migration_function (database_id,
                             std::function&lt;data_migration_function_type>,
                             const std::string&amp; name = "");
  };

  // Static data migration function registration, C++98/03 version:
  //
  template &lt;schema_version v, schema_version base>
  struct data_migration_entry
  {
    data_migration_entry (data_migration_function_type*,
                          const std::string&amp; name = "");

    data_migration_entry (database_id,
                          data_migration_function_type*,
                          const std::string&amp; name = "");
  };

  // Static data migration function registration, C++11 version:
  //
  template &lt;schema_version v, schema_version base>
  struct data_migration_entry
  {
    data_migration_entry (std::function&lt;data_migration_function_type>,
                          const std::string&amp; name = "");

    data_migration_entry (database_id,
                          std::function&lt;data_migration_function_type>,
                          const std::string&amp; name = "");
  };
}
  </pre>

  <p>The <code>migrate_data()</code> static function performs data
     migration for the specified version. If no version is specified,
     then it will use the current database version and also check
     whether the database is in migration, that is,
     <code>database::schema_migration()</code> returns <code>true</code>.
     As a result, all we need to do in our <code>main()</code> is call
     this function. It will check if migration is required and if so,
     call all the migration functions registered for this version. For
     example:</p>

  <pre class="cxx">
int
main ()
{
  ...

  database&amp; db = ...

  // Check if we need to migrate any data and do so
  // if that's the case.
  //
  schema_catalog::migrate_data (db);

  ...
}
  </pre>

  <p>The <code>migrate_data()</code> function returns the number of
     migration functions called. You can use this value for debugging
     or logging.</p>

  <p>The only other step that we need to perform is register our data
     migration functions with <code>schema_catalog</code>. At the
     lower level we can call the <code>data_migration_function()</code>
     static function for every migration function we have, for example,
     at the beginning of <code>main()</code>. For each version, data
     migration functions are called in the order of registration.</p>

  <p>A more convenient approach, however, is to use the
     <code>data_migration_entry</code> helper class template to register the
     migration functions during static initialization. This way we
     can keep the migration function and its registration code next
     to each other. Here is how we can reimplement our <code>gender</code>
     migration code to use this mechanism:</p>

  <pre class="cxx">
static void
migrate_gender (odb::database&amp; db)
{
  transaction t (db.begin ());

  for (person&amp; p: db.query&lt;person> ())
  {
    p.gender (guess_gender (p.first ()));
    db.update (p);
  }

  t.commit ();
}

static const odb::data_migration_entry&lt;3, MYAPP_BASE_VERSION>
migrate_gender_entry (&amp;migrate_gender);
  </pre>

  <p>The first template argument to the <code>data_migration_entry</code>
     class template is the version we want this data migration function
     to be called for. The second template argument is the base model
     version. This second argument is necessary to detect the situation
     where we no longer need this data migration function. Remember
     that when we move the base model version forward, migrations from
     any version below the new base are no longer possible. We, however,
     may still have migration functions registered for those lower
     versions. Since these functions will never be called, they are
     effectively dead code and it would be useful to identify and
     remove them. To assist with this, <code>data_migration_entry</code>
     (and lower lever <code>data_migration_function()</code>) will
     check at compile time (that is, <code>static_assert</code>) that
     the registration version is greater than the base model version.</p>

  <p>In the above example we use the <code>MYAPP_BASE_VERSION</code>
     macro that is presumably defined in a central place, for example,
     <code>version.hxx</code>. This is the recommended approach since
     we can update the base version in a single place and have the
     C++ compiler automatically identify all the data migration
     functions that can be removed.</p>

  <p>In C++11 we can also create a template alias so that we don't
     have to repeat the base model macro in every registration, for
     example:</p>

  <pre class="cxx">
template &lt;schema_version v>
using migration_entry = odb::data_migration_entry&lt;v, MYAPP_BASE_VERSION>;

static const migration_entry&lt;3>
migrate_gender_entry (&amp;migrate_gender);
  </pre>

  <p>For cases where you need to by-pass the base version check, for
     example, to implement your own registration helper, ODB also
     provides "unsafe" versions of the <code>data_migration_function()</code>
     functions that take the version as a function argument rather than
     as a template parameter.</p>

  <p>In C++11 we can also use lambdas as migration functions, which makes
     the migration code more concise:</p>

  <pre class="cxx">
static const migration_entry&lt;3>
migrate_gender_entry (
  [] (odb::database&amp; db)
  {
    transaction t (db.begin ());

    for (person&amp; p: db.query&lt;person> ())
    {
      p.gender (guess_gender (p.first ()));
      db.update (p);
    }

    t.commit ();
  });
  </pre>

  <p>If we are using embedded schema migrations, then both schema and
     data migration is integrated and can be performed with a single
     call to the <code>schema_catalog::migrate()</code> function that
     we discussed earlier. For example:</p>

<pre class="cxx">
int
main ()
{
  ...

  database&amp; db = ...

  // Check if we need to migrate the database and do so
  // if that's the case.
  //
  {
    transaction t (db.begin ());
    schema_catalog::migrate (db);
    t.commit ();
  }

  ...
}
  </pre>

  <p>Note, however, that in this case we call <code>migrate()</code>
     within a transaction (for the schema migration part) which means
     that our migration functions will also be called within this
     transaction. As a result, we will need to adjust our migration
     functions not to start their own transaction:</p>

  <pre class="cxx">
static void
migrate_gender (odb::database&amp; db)
{
  // Assume we are already in a transaction.
  //
  for (person&amp; p: db.query&lt;person> ())
  {
    p.gender (guess_gender (p.first ()));
    db.update (p);
  }
}
  </pre>

  <p>If, however, we want more granular transactions, then we can
     use the lower-level <code>schema_catalog</code> functions to
     gain more control, as we have seen at the end of the previous
     section.  Here is the relevant part of that example with
     an added data migration call:</p>

  <pre class="cxx">
  // Old schema (and data) in the database, migrate them.
  //
  for (v = schema_catalog::next_version (db, v);
       v &lt;= cv;
       v = schema_catalog::next_version (db, v))
  {
    transaction t (db.begin ());
    schema_catalog::migrate_schema_pre (db, v);
    schema_catalog::migrate_data (db, v);
    schema_catalog::migrate_schema_post (db, v);
    t.commit ();
  }
  </pre>

  <h2><a name="13.3.2">13.3.2 Gradual Data Migration</a></h2>

  <p>If the number of existing objects that require migration is large,
     then an all-at-once, immediate migration, while simple, may not
     be practical from a performance point of view. In this case,
     we can perform a gradual migration as the application does
     its normal functions.</p>

  <p>With gradual migrations, the object model must be capable of
     representing data that conforms to both old and new formats at
     the same time since, in general, the database will contain a
     mixture of old and new objects. For example, in case of our
     <code>gender</code> data member, we need a special value that
     represents the "no gender assigned yet" case (an old object).
     We also need to assign this special value to all the existing
     objects during the schema pre-migration stage. One way to do
     this would be add a special value to our <code>gender</code>
     enum and then make it the default value with the
     <code>db&nbsp;default</code> pragma. A cleaner and easier approach,
     however, is to use <code>NULL</code> as a special value. We
     can add support for the <code>NULL</code> value semantics
     to any existing type by wrapping it with
     <code>odb::nullable</code>, <code>boost::optional</code>
     or similar (<a href="#7.3">Section 7.3, "Pointers and <code>NULL</code>
     Value Semantics"</a>). We also don't need to specify the default value
     explicitly since <code>NULL</code> is used automatically. Here
     is how we can use this approach in our <code>gender</code>
     example:</p>

  <pre class="cxx">
#include &lt;odb/nullable.hxx>

#pragma db object
class person
{
  ...

  odb::nullable&lt;gender> gender_;
};
  </pre>

  <p>A variety of strategies can be employed to implement gradual
     migrations. For example, we can migrate the data when the object
     is updated as part of the normal application logic. While there
     is no migration cost associated with this approach (the object
     is updated anyway), depending on how often objects are typically
     updated, this strategy can take a long time to complete. An
     alternative strategy would be to perform an update whenever
     an old object is loaded. Yet another strategy is to have a
     separate thread that slowly migrates all the old objects as
     the application runs.</p>

  <p>As an example, let us implement the first approach for our
     <code>gender</code> migration. While we could have added
     the necessary code throughout the application, from the
     maintenance point of view, it is best to try and localize
     the gradual migration logic to the persistent classes that
     it affects. And for this database operation callbacks
     (<a href="#14.1.7">Section 14.1.7, "<code>callback</code>"</a>)
     are a very useful mechanism. In our case, all we have to do is handle
     the <code>post_load</code> event where we guess the gender
     if it is <code>NULL</code>:</p>

  <pre class="cxx">
#include &lt;odb/core.hxx>     // odb::database
#include &lt;odb/callback.hxx> // odb::callback_event
#include &lt;odb/nullable.hxx>

#pragma db object callback(migrate)
class person
{
  ...

  void
  migrate (odb::callback_event e, odb::database&amp;)
  {
    if (e == odb::callback_event::post_load)
    {
      // Guess gender if not assigned.
      //
      if (gender_.null ())
        gender_ = guess_gender (first_);
    }
  }

  odb::nullable&lt;gender> gender_;
};
  </pre>

  <p>In particular, we don't have to touch any of the accessors
     or modifiers or the application logic &mdash; all of them
     can assume that the value can never be <code>NULL</code>.
     And when the object is next updated, the new <code>gender</code>
     value will be stored automatically.</p>

  <p>All gradual migrations normally end up with a terminating
     immediate migration some number of versions down the line,
     when the bulk of the objects has presumably been converted.
     This way we don't have to keep the gradual migration code
     around forever. Here is how we could implement a terminating
     migration for our example:</p>

  <pre class="cxx">
// person.hxx
//
#pragma db model version(1, 4)

#pragma db object
class person
{
  ...

  gender gender_;
};

// person.cxx
//
static void
migrate_gender (odb::database&amp; db)
{
  using query = odb::query&lt;person>;

  for (person&amp; p: db.query&lt;person> (query::gender.is_null ()))
  {
    p.gender (guess_gender (p.first ()));
    db.update (p);
  }
}

static const odb::data_migration_entry&lt;4, MYAPP_BASE_VERSION>
migrate_gender_entry (&amp;migrate_gender);
  </pre>

  <p>A couple of points to note about this code. Firstly, we
     removed all the gradual migration logic (the callback)
     from the class and replaced it with the immediate migration
     function. We also removed the <code>odb::nullable</code>
     wrapper (and therefore disallowed the <code>NULL</code> values)
     since after this migration all the objects will have been
     converted. Finally, in the migration function, we only query
     the database for objects that need migration, that is, have
     <code>NULL</code> gender.</p>

  <h2><a name="13.4">13.4 Soft Object Model Changes</a></h2>

  <p>Let us consider another common kind of object model change:
     we delete an old member, add a new one, and need to copy
     the data from the old to the new, perhaps applying some
     conversion. For example, we may realize that in our application
     it is a better idea to store a person's name as a single string
     rather than split it into three fields. So what we would like to do
     is add a new data member, let's call it <code>name_</code>, convert
     all the existing split names, and then delete the <code>first_</code>,
     <code>middle_</code>, and <code>last_</code> data members.</p>

  <p>While this sounds straightforward, there is a problem. If we
     delete (that is, physically remove from the source code) the
     old data members, then we won't be able to access the old
     data. The data will still be available in the database between
     the schema pre and post-migrations, it is just we will no longer
     be able to access it through our object model. And if we keep
     the old data members around, then the old data will remain
     stored in the database even after the schema post-migration.</p>

  <p>There is also a more subtle problem that has to do with existing
     migrations for the previous versions. Remember, in version <code>3</code>
     of our <code>person</code> example we added the <code>gender_</code>
     data member. We also have a data migration function which guesses
     the gender based on the first name. Deleting the <code>first_</code>
     data member from our class will obviously break this code. But
     even adding the new <code>name_</code> data member will cause
     problems because when we try to update the object in order to
     store the new gender, ODB will try to update <code>name_</code>
     as well. But there is no corresponding column in the database
     yet. When we run this migration function, we are still several
     versions away from the point where the <code>name</code> column
     will be added.</p>

  <p>This is a very subtle but also very important implication to
     understand. Unlike the main application logic, which only needs
     to deal with the current model version, data migration code works
     on databases that can be multiple versions behind the current
     version.</p>

  <p>How can we resolve this problem? It appears what we need is the
     ability to add or delete data members starting from a specific
     version. In ODB this mechanism is called soft member additions
     and deletions. A soft-added member is only treated as persistent
     starting from the addition version. A soft-deleted member is
     persistent until the deletion version (but including the migration
     stage). In its essence, soft model changes allow us to maintain
     multiple versions of our object model all with a single set of
     persistent classes. Let us now see how this functionality can
     help implement our changes:</p>

  <pre class="cxx">
#pragma db model version(1, 4)

#pragma db object
class person
{
  ...

  #pragma db id auto
  unsigned long long id_;

  #pragma db deleted(4)
  std::string first_;

  #pragma db deleted(4)
  std::string middle_;

  #pragma db deleted(4)
  std::string last_;

  #pragma db added(4)
  std::string name_;

  gender gender_;
};
  </pre>

  <p>The migration function for this change could then look like
     this:</p>

  <pre class="cxx">
static void
migrate_name (odb::database&amp; db)
{
  for (person&amp; p: db.query&lt;person> ())
  {
    p.name (p.first () + " " +
            p.middle () + (p.middle ().empty () ? "" : " ") +
            p.last ());
    db.update (p);
  }
}

static const odb::data_migration_entry&lt;4, MYAPP_BASE_VERSION>
migrate_name_entry (&amp;migrate_name);
  </pre>

  <p>Note also that no changes are required to the gender migration
     function.</p>

  <p>As you may have noticed, in the code above we assumed that the
     <code>person</code> class still provides public accessors for
     the now deleted data members. This might not be ideal since now
     they should not be used by the application logic. The only code
     that may still need to access them is the migration functions. The
     recommended way to resolve this is to remove the accessors/modifiers
     corresponding to the deleted data member, make migration functions
     static functions of the class being migrated, and then access
     the deleted data members directly. For example:</p>

  <pre class="cxx">
#pragma db model version(1, 4)

#pragma db object
class person
{
  ...

private:
  friend class odb::access;

  #pragma db id auto
  unsigned long long id_;

  #pragma db deleted(4)
  std::string first_;

  #pragma db deleted(4)
  std::string middle_;

  #pragma db deleted(4)
  std::string last_;

  #pragma db added(4)
  std::string name_;

  gender gender_;

private:
  static void
  migrate_gender (odb::database&amp;);

  static void
  migrate_name (odb::database&amp;);
};

void person::
migrate_gender (odb::database&amp; db)
{
  for (person&amp; p: db.query&lt;person> ())
  {
    p.gender_ = guess_gender (p.first_);
    db.update (p);
  }
}

static const odb::data_migration_entry&lt;3, MYAPP_BASE_VERSION>
migrate_name_entry (&amp;migrate_gender);

void person::
migrate_name (odb::database&amp; db)
{
  for (person&amp; p: db.query&lt;person> ())
  {
    p.name_ = p.first_ + " " +
              p.middle_ + (p.middle_.empty () ? "" : " ") +
              p.last_;
    db.update (p);
  }
}

static const odb::data_migration_entry&lt;4, MYAPP_BASE_VERSION>
migrate_name_entry (&amp;migrate_name);
  </pre>

  <p>Another potential issue with the soft-deletion is the requirement
     to keep the delete data members in the class. While they will not
     be initialized in the normal operation of the application (that
     is, not a migration), this can still be a problem if we need to
     minimize the memory footprint of our classes. For example, we may
     cache a large number of objects in memory and having three
     <code>std::string</code> data members can be a significant
     overhead.</p>

  <p>The recommended way to resolve this issue is to place all the
     deleted data members into a dynamically allocated composite
     value type. For example:</p>

  <pre class="cxx">
#pragma db model version(1, 4)

#pragma db object
class person
{
  ...

  #pragma db id auto
  unsigned long long id_;

  #pragma db added(4)
  std::string name_;

  gender gender_;

  #pragma db value
  struct deleted_data
  {
    #pragma db deleted(4)
    std::string first_;

    #pragma db deleted(4)
    std::string middle_;

    #pragma db deleted(4)
    std::string last_;
  };

  #pragma db column("")
  std::unique_ptr&lt;deleted_data> dd_;

  ...
};
  </pre>

  <p>ODB will then automatically allocate the deleted value type if
     any of the deleted data members are being loaded. During the normal
     operation, however, the pointer will stay <code>NULL</code> and
     therefore reduce the common case overhead to a single pointer
     per class. Note that we make the composite value column prefix
     empty (the <code>db&nbsp;column("")</code> pragma) in order to
     keep the same column names for the deleted data members.</p>

  <p>Soft-added and deleted data members can be used in objects,
     composite values, views, and container value types. We can
     also soft-add and delete data members of simple, composite,
     pointer to object, and container types. Only special data
     members, such as the object id and the optimistic concurrency
     version, cannot be soft-added or deleted.</p>

  <p>It is also possible to soft-delete a persistent class. We
     can still work with the existing objects of such a class,
     however, no table is created in new databases for soft-deleted
     classes. To put it another way, a soft-delete class is like an
     abstract class (no table) but which can still be loaded, updated,
     etc. Soft-added persistent classes do not make much sense and
     are therefore not supported.</p>

  <p>As an example of a soft-deleted class, suppose we want to
     replace our <code>person</code> class with the new
     <code>employee</code> object and migrate the data. Here is
     how we could do this:</p>

  <pre class="cxx">
#pragma db model version(1, 5)

#pragma db object deleted(5)
class person
{
  ...
};

#pragma db object
class employee
{
  ...

  #pragma db id auto
  unsigned long long id_;

  std::string name_;
  gender gender_;

  static void
  migrate_person (odb::database&amp;);
};

void employee::
migrate_person (odb::database&amp; db)
{
  for (person&amp; p: db.query&lt;person> ())
  {
    employee e (p.name (), p.gender ());
    db.persist (e);
  }
}

static const odb::data_migration_entry&lt;5, MYAPP_BASE_VERSION>
migrate_person_entry (&amp;migrate_person);
  </pre>

  <p>As we have seen above, hard member additions and deletions can
     (and most likely will) break existing data migration code. Why,
     then, not treat all the changes, or at least additions, as soft?
     ODB requires you to explicitly request this semantics because
     support for soft-added and deleted data members incurs runtime
     overhead. And there can be plenty of cases where there is no
     existing data migration and therefore hard additions and deletions
     are sufficient.</p>

  <p>In some cases a hard addition or deletion will result in a
     compile-time error. For example, one of the data migration
     functions may reference the data member we just deleted. In
     many cases, however, such errors can only be detected at
     runtime, and, worse yet, only when the migration function
     is executed. For example, we may hard-add a new data member
     that an existing migration function will try to indirectly
     store in the database as part of an object update. As a result,
     it is highly recommended that you always test your application
     with the database that starts at the base version so that every
     data migration function is called and therefore ensured to
     still work correctly.</p>

  <p>To help with this problem you can also instruct ODB to warn
     you about any hard additions or deletions with the
     <code>--warn-hard-add</code>, <code>--warn-hard-delete</code>,
     and <code>--warn-hard</code> command line options. ODB will
     only warn you about hard changes in the current version and
     only for as long as it is open, which makes this mechanism
     fairly usable.</p>

  <p>You may also be wondering why we have to specify the addition
     and deletion versions explicitly. It may seem like the ODB compiler
     should be able to figure this out automatically. While it is
     theoretically possible, to achieve this, ODB would have to also
     maintain a separate changelog of the C++ object model in
     addition to the database schema changelog it already maintains.
     While being a lot more complex, such an additional changelog
     would also complicate the workflow significantly. In this light,
     maintaining this change information as part of the original
     source files appears to be a cleaner and simpler approach.</p>

  <p>As we discussed before, when we move the base model version
     forward we essentially drop support for migrations from
     versions before the new base. As a result, it is no longer
     necessary to maintain the soft semantics of additions and
     deletions up to and including the new base version. ODB
     will issue diagnostics for all such members and classes.
     For soft deletions we can simply remove the data member or
     class entirely. For soft additions we only need to remove the
     <code>db&nbsp;added</code> pragma.</p>

  <h2><a name="13.4.1">13.4.1 Reuse Inheritance Changes</a></h2>

  <p>Besides adding and deleting data members, another way to alter
     the object's table is using reuse-style inheritance. If we add
     a new reuse base, then, from the database schema point of view,
     this is equivalent to adding all its columns to the derived
     object's table. Similarly, deleting reuse inheritance results in
     all the base's columns being deleted from the derived's table.</p>

  <p>In the future ODB may provide direct support for soft addition
     and deletion of inheritance. Currently, however, this semantics
     can be emulated with soft-added and deleted data members. The
     following table describes the most common scenarios depending
     on where columns are added or deleted, that is, base table,
     derived table, or both.</p>

  <!-- border="1" is necessary for html2ps -->
  <table class="scenarios" border="1">
    <tr>
      <th>DELETE</th>
      <th style="width: 40%">HARD</th>
      <th style="width: 40%">SOFT</th>
    </tr>

    <tr>
      <td>In both (delete inheritance and base)</td>
      <td>Delete inheritance and base. Move object id to derived.</td>
      <td>Soft-delete base. Mark all data members (except id) in
          base as soft-deleted.</td>
    </tr>

    <tr>
      <td>In base only (delete base)</td>
      <td>Option 1: mark base as abstract.<br/><br/>
          Option 2: move all the base member to derived, delete base.</td>
      <td>Soft-delete base.</td>
    </tr>

    <tr>
      <td>In derived only (delete inheritance)</td>
      <td>Delete inheritance, add object id to derived.</td>
      <td>Option 1: copy base to a new soft-deleted base, inherit
          from it instead. Mark all the data members (expect id) in
          this new base as soft-deleted. Note: we add the new base
          as soft-deleted to get notified when we can remove it.<br/><br/>
          Option 2: Copy all the data members from base to derived
          and mark them as soft-deleted in derived.</td>
    </tr>
  </table>


  <table class="scenarios" border="1">
    <tr>
      <th>ADD</th>
      <th style="width: 40%">HARD</th>
      <th style="width: 40%">SOFT</th>
    </tr>

    <tr>
      <td>In both (add new base and inheritance)</td>
      <td>Add new base and inheritance. Potentially move object id
          member from derived to base.</td>
      <td>Add new base and mark all its data members as soft-added.
          Add inheritance. Move object id from derived to base.</td>
    </tr>

    <tr>
      <td>In base only (refactor existing data to new base)</td>
      <td>Add new base and move data members from derived to base.
          Note: in most cases the new base will be made abstract
          which make this scenario non-schema changing.</td>
      <td>The same as HARD.</td>
    </tr>

    <tr>
      <td>In derived only (add inheritance to existing base)</td>
      <td>Add inheritance, delete object id in derived.</td>
      <td>Copy existing base to a new abstract base and inherit
          from it. Mark all the database members in the new base
          as soft-added (except object id). When notified by the
          ODB compiler that the soft addition of the data members
          is no longer necessary, delete the copy and inherit from
          the original base.</td>
    </tr>
  </table>

  <h2><a name="13.4.2">13.4.2 Polymorphism Inheritance Changes</a></h2>

  <p>Unlike reuse inheritance, adding or deleting a polymorphic base
     does not result in the base's data members being added or deleted
     from the derived object's table because each class in a polymorphic
     hierarchy is stored in a separate table. There are, however, other
     complications due to the presence of special columns (discriminator
     in the root table and object id links in derived tables) which makes
     altering the hierarchy structure difficult to handle automatically.
     Adding or deleting (including soft-deleting) of leaf classes (or
     leaf sub-hierarchies) in a polymorphic hierarchy is fully supported.
     Any more complex changes, such as adding or deleting the root or
     an intermediate base or getting an existing class into or out of
     a polymorphic hierarchy can be handled by creating a new leaf class
     (or leaf sub-hierarchy), soft-deleting the old class, and migrating
     the data.</p>

  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="14">14 ODB Pragma Language</a></h1>

  <p>As we have already seen in previous chapters, ODB uses a pragma-based
     language to capture database-specific information about C++ types.
     This chapter describes the ODB pragma language in more detail. It
     can be read together with other chapters in the manual to get a
     sense of what kind of configurations and mapping fine-tuning are
     possible. You can also use this chapter as a reference at a later
     stage.</p>

  <p>An ODB pragma has the following syntax:</p>

  <p><code>#pragma db <i>qualifier</i> [<i>specifier</i> <i>specifier</i> ...]</code></p>

  <p>The <em>qualifier</em> tells the ODB compiler what kind of C++ construct
     this pragma describes. Valid qualifiers are <code>object</code>,
     <code>view</code>, <code>value</code>, <code>member</code>,
     <code>namespace</code>, <code>model</code>, <code>index</code>, and
     <code>map</code>.
     A pragma with the <code>object</code> qualifier describes a persistent
     object type. It tells the ODB compiler that the C++ class it describes
     is a persistent class. Similarly, pragmas with the <code>view</code>
     qualifier describe view types, the <code>value</code> qualifier
     describes value types and the <code>member</code> qualifier is used
     to describe data members of persistent object, view, and value types.
     The <code>namespace</code> qualifier is used to describe common
     properties of objects, views, and value types that belong to
     a C++ namespace while the <code>model</code> qualifier describes
     the whole C++ object model. The <code>index</code> qualifier defines
     a database index. And, finally, the <code>map</code> qualifier
     describes a mapping between additional database types and types
     for which ODB provides built-in support.</p>

  <p>The <em>specifier</em> informs the ODB compiler about a particular
     database-related property of the C++ declaration. For example, the
     <code>id</code> member specifier tells the ODB compiler that this
     member contains this object's identifier. Below is the declaration
     of the <code>person</code> class that shows how we can use ODB
     pragmas:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
private:
  #pragma db member id
  unsigned long long id_;
  ...
};
  </pre>

  <p>In the above example we don't explicitly specify which C++ class or
     data member the pragma belongs to. Rather, the pragma applies to
     a C++ declaration that immediately follows the pragma. Such pragmas
     are called <em>positioned pragmas</em>. In positioned pragmas that
     apply to data members, the <code>member</code> qualifier can be
     omitted for brevity, for example:</p>

  <pre class="cxx">
  #pragma db id
  unsigned long long id_;
  </pre>

  <p>Note also that if the C++ declaration immediately following a
     position pragma is incompatible with the pragma qualifier, an
     error will be issued. For example:</p>

  <pre class="cxx">
  #pragma db object  // Error: expected class instead of data member.
  unsigned long long id_;
  </pre>

  <p>While keeping the C++ declarations and database declarations close
     together eases maintenance and increases readability, we can also
     place them in different parts of the same header file or even
     factor them to a separate file. To achieve this we use the so called
     <em>named pragmas</em>. Unlike positioned pragmas, named pragmas
     explicitly specify the C++ declaration to which they apply by
     adding the declaration name after the pragma qualifier. For example:</p>

  <pre class="cxx">
class person
{
  ...
private:
  unsigned long long id_;
  ...
};

#pragma db object(person)
#pragma db member(person::id_) id
  </pre>

  <p>Note that in the named pragmas for data members the <code>member</code>
     qualifier is no longer optional. The C++ declaration name in the
     named pragmas is resolved using the standard C++ name resolution
     rules, for example:</p>

  <pre class="cxx">
namespace db
{
  class person
  {
    ...
  private:
    unsigned long long id_;
    ...
  };
}

namespace db
{
  #pragma db object(person)  // Resolves db::person.
}

#pragma db member(db::person::id_) id
  </pre>

  <p>As another example, the following code fragment shows how to use the
     named value type pragma to map a C++ type to a native database type:</p>

  <pre class="cxx">
#pragma db value(bool) type("INT")

#pragma db object
class person
{
  ...
private:
  bool married_; // Mapped to INT NOT NULL database type.
  ...
};
  </pre>

  <p>If we would like to factor the ODB pragmas into a separate file,
     we can include this file into the original header file (the one
     that defines the persistent types) using the <code>#include</code>
     directive, for example:</p>

  <pre class="cxx">
// person.hxx

class person
{
  ...
};

#ifdef ODB_COMPILER
#  include "person-pragmas.hxx"
#endif
  </pre>

  <p>Alternatively, instead of using the <code>#include</code> directive,
     we can use the <code>--odb-epilogue</code> option to make the pragmas
     known to the ODB compiler when compiling the original header file,
     for example:</p>

  <pre class="terminal">
--odb-epilogue  '#include "person-pragmas.hxx"'
  </pre>

  <p>The following sections cover the specifiers applicable to all the
     qualifiers mentioned above.</p>

  <p>The C++ header file that defines our persistent classes and
     normally contains one or more ODB pragmas is compiled by both
     the ODB compiler to generate the database support code and
     the C++ compiler to build the application. Some C++ compilers
     issue warnings about pragmas that they do not recognize. There
     are several ways to deal with this problem which are covered
     at the end of this chapter in <a href="#14.9">Section 14.9,
     "C++ Compiler Warnings"</a>.</p>

  <h2><a name="14.1">14.1 Object Type Pragmas</a></h2>

  <p>A pragma with the <code>object</code> qualifier declares a C++ class
     as a persistent object type. The qualifier can be optionally followed,
     in any order, by one or more specifiers summarized in the table below:</p>

  <!-- border="1" is necessary for html2ps -->
  <table class="specifiers" border="1">
    <tr>
      <th>Specifier</th>
      <th>Summary</th>
      <th>Section</th>
    </tr>

    <tr>
      <td><code>table</code></td>
      <td>table name for a persistent class</td>
      <td><a href="#14.1.1">14.1.1</a></td>
    </tr>

    <tr>
      <td><code>pointer</code></td>
      <td>pointer type for a persistent class</td>
      <td><a href="#14.1.2">14.1.2</a></td>
    </tr>

    <tr>
      <td><code>abstract</code></td>
      <td>persistent class is abstract</td>
      <td><a href="#14.1.3">14.1.3</a></td>
    </tr>

    <tr>
      <td><code>readonly</code></td>
      <td>persistent class is read-only</td>
      <td><a href="#14.1.4">14.1.4</a></td>
    </tr>

    <tr>
      <td><code>optimistic</code></td>
      <td>persistent class with the optimistic concurrency model</td>
      <td><a href="#14.1.5">14.1.5</a></td>
    </tr>

    <tr>
      <td><code>no_id</code></td>
      <td>persistent class has no object id</td>
      <td><a href="#14.1.6">14.1.6</a></td>
    </tr>

    <tr>
      <td><code>callback</code></td>
      <td>database operations callback</td>
      <td><a href="#14.1.7">14.1.7</a></td>
    </tr>

    <tr>
      <td><code>schema</code></td>
      <td>database schema for a persistent class</td>
      <td><a href="#14.1.8">14.1.8</a></td>
    </tr>

    <tr>
      <td><code>polymorphic</code></td>
      <td>persistent class is polymorphic</td>
      <td><a href="#14.1.9">14.1.9</a></td>
    </tr>

    <tr>
      <td><code>session</code></td>
      <td>enable/disable session support for a persistent class</td>
      <td><a href="#14.1.10">14.1.10</a></td>
    </tr>

    <tr>
      <td><code>definition</code></td>
      <td>definition location for a persistent class</td>
      <td><a href="#14.1.11">14.1.11</a></td>
    </tr>

    <tr>
      <td><code>transient</code></td>
      <td>all non-virtual data members in a persistent class are transient</td>
      <td><a href="#14.1.12">14.1.12</a></td>
    </tr>

    <tr>
      <td><code>sectionable</code></td>
      <td>support addition of new sections in derived classes</td>
      <td><a href="#14.1.13">14.1.13</a></td>
    </tr>

    <tr>
      <td><code>deleted</code></td>
      <td>persistent class is soft-deleted</td>
      <td><a href="#14.1.14">14.1.14</a></td>
    </tr>

    <tr>
      <td><code>bulk</code></td>
      <td>enable bulk operations for a persistent class</td>
      <td><a href="#14.1.15">14.1.15</a></td>
    </tr>

    <tr>
      <td><code>options</code></td>
      <td>database options for a persistent class</td>
      <td><a href="#14.1.16">14.1.16</a></td>
    </tr>

  </table>

  <h3><a name="14.1.1">14.1.1 <code>table</code></a></h3>

  <p>The <code>table</code> specifier specifies the table name that should
     be used to store objects of the persistent class in a relational
     database. For example:</p>

  <pre class="cxx">
#pragma db object table("people")
class person
{
  ...
};
  </pre>

  <p>If the table name is not specified, the class name is used as the
     table name. The table name can be qualified with a database
     schema, for example:</p>

  <pre class="cxx">
#pragma db object table("census.people")
class person
{
  ...
};
  </pre>

  <p>For more information on database schemas and the format of the
     qualified names, refer to <a href="#14.1.8">Section 14.1.8,
     "<code>schema</code>"</a>.</p>

  <h3><a name="14.1.2">14.1.2 <code>pointer</code></a></h3>

  <p>The <code>pointer</code> specifier specifies the object pointer type
     for the persistent class. The object pointer type is used to return,
     pass, and cache dynamically allocated instances of a persistent
     class. For example:</p>

  <pre class="cxx">
#pragma db object pointer(std::shared_ptr&lt;person>)
class person
{
  ...
};
  </pre>

  <p>There are several ways to specify an object pointer with the
     <code>pointer</code> specifier. We can use a complete pointer
     type as shown in the example above. Alternatively, we can
     specify only the template name of a smart pointer in which
     case the ODB compiler will automatically append the class
     name as a template argument. The following example is therefore
     equivalent to the one above:</p>

  <pre class="cxx">
#pragma db object pointer(std::shared_ptr)
class person
{
  ...
};
  </pre>

  <p>If you would like to use the raw pointer as an object pointer,
     you can use <code>*</code> as a shortcut:</p>

  <pre class="cxx">
#pragma db object pointer(*) // Same as pointer(person*)
class person
{
  ...
};
  </pre>

  <p>If a pointer type is not explicitly specified, the default pointer,
     specified at the namespace level (<a href="#14.5.1">Section 14.5.1,
     "<code>pointer</code>"</a>) or with the <code>--default-pointer</code>
     ODB compiler option, is used. If neither of these two mechanisms is
     used to specify the pointer, then the raw pointer is used by default.</p>

  <p>For a more detailed discussion of object pointers, refer to
     <a href="#3.3">Section 3.3, "Object and View Pointers"</a>.</p>

  <h3><a name="14.1.3">14.1.3 <code>abstract</code></a></h3>

  <p>The <code>abstract</code> specifier specifies that the persistent class
     is abstract. An instance of an abstract class cannot be stored in
     the database and is normally used as a base for other persistent
     classes. For example:</p>

  <pre class="cxx">
#pragma db object abstract
class person
{
  ...
};

#pragma db object
class employee: public person
{
  ...
};

#pragma db object
class contractor: public person
{
  ...
};
  </pre>

  <p>Persistent classes with pure virtual functions are automatically
     treated as abstract by the ODB compiler. For a more detailed
     discussion of persistent class inheritance, refer to
     <a href="#8">Chapter 8, "Inheritance"</a>.</p>

  <h3><a name="14.1.4">14.1.4 <code>readonly</code></a></h3>

  <p>The <code>readonly</code> specifier specifies that the persistent class
     is read-only. The database state of read-only objects cannot be
     updated. In particular, this means that you cannot call the
     <code>database::update()</code> function (<a href="#3.10">Section 3.10,
     "Updating Persistent Objects"</a>) for such objects. For example:</p>

  <pre class="cxx">
#pragma db object readonly
class person
{
  ...
};
  </pre>

  <p>Read-only and read-write objects can derive from each other without
     any restrictions. When a read-only object derives from a read-write
     object, the resulting whole object is read-only, including the part
     corresponding to the read-write base. On the other hand, when a
     read-write object derives from a read-only object, all the data
     members that correspond to the read-only base are treated as
     read-only while the rest is treated as read-write.</p>

  <p>Note that it is also possible to declare individual data members
    (<a href="#14.4.12">Section 14.4.12, "<code>readonly</code>"</a>)
     as well as composite value types (<a href="#14.3.6">Section 14.3.6,
     "<code>readonly</code>"</a>) as read-only.</p>

  <h3><a name="14.1.5">14.1.5 <code>optimistic</code></a></h3>

  <p>The <code>optimistic</code> specifier specifies that the persistent class
     has the optimistic concurrency model. A class with the optimistic
     concurrency model must also specify the data member that is used to
     store the object version using the <code>version</code> pragma
     (<a href="#14.4.16">Section 14.4.16, "<code>version</code>"</a>).
     For example:</p>

  <pre class="cxx">
#pragma db object optimistic
class person
{
  ...

  #pragma db version
  unsigned long long version_;
};
  </pre>

  <p>If a base class has the optimistic concurrency model, then all its derived
     classes will automatically have the optimistic concurrency model. The
     current implementation also requires that in any given inheritance
     hierarchy the object id and the version data members reside in the
     same class.</p>

  <p>For a more detailed discussion of optimistic concurrency, refer to
     <a href="#12">Chapter 12, "Optimistic Concurrency"</a>.</p>

  <h3><a name="14.1.6">14.1.6 <code>no_id</code></a></h3>

  <p>The <code>no_id</code> specifier specifies that the persistent class
     has no object id. For example:</p>

  <pre class="cxx">
#pragma db object no_id
class person
{
  ...
};
  </pre>

  <p>A persistent class without an object id has limited functionality.
     Such a class cannot be loaded with the <code>database::load()</code>
     or <code>database::find()</code> functions (<a href="#3.9">Section 3.9,
     "Loading Persistent Objects"</a>), updated with the
     <code>database::update()</code> function (<a href="#3.10">Section 3.10,
     "Updating Persistent Objects"</a>), or deleted with the
     <code>database::erase()</code> function (<a href="#3.11">Section 3.11,
     "Deleting Persistent Objects"</a>). To load and delete
     objects without ids you can use the <code>database::query()</code>
     (<a href="#4">Chapter 4, "Querying the Database"</a>) and
     <code>database::erase_query()</code> (<a href="#3.11">Section 3.11,
     "Deleting Persistent Objects"</a>) functions, respectively.
     There is no way to update such objects except by using native SQL
     statements (<a href="#3.12">Section 3.12, "Executing Native SQL
     Statements"</a>).</p>

  <p>Furthermore, persistent classes without object ids cannot have container
     data members nor can they be used in object relationships. Such objects
     are not entered into the session object cache
     (<a href="#11.1">Section 11.1, "Object Cache"</a>) either.</p>

  <p>To declare a persistent class with an object id, use the data member
     <code>id</code> specifier (<a href="#14.4.1">Section 14.4.1,
     "<code>id</code>"</a>).</p>

  <h3><a name="14.1.7">14.1.7 <code>callback</code></a></h3>

  <p>The <code>callback</code> specifier specifies the persist class
     member function that should be called before and after a
     database operation is performed on an object of this class.
     For example:</p>

  <pre class="cxx">
#include &lt;odb/callback.hxx>

#pragma db object callback(init)
class person
{
  ...

  void
  init (odb::callback_event, odb::database&amp;);
};
 </pre>

  <p>The callback function has the following signature and can be
     overloaded for constant objects:</p>

  <pre class="cxx">
void
name (odb::callback_event, odb::database&amp;);

void
name (odb::callback_event, odb::database&amp;) const;
  </pre>

  <p>The first argument to the callback function is the event that
     triggered this call. The <code>odb::callback_event</code>
     enum-like type is defined in the <code>&lt;odb/callback.hxx></code>
     header file and has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  struct callback_event
  {
    enum value
    {
      pre_persist,
      post_persist,
      pre_load,
      post_load,
      pre_update,
      post_update,
      pre_erase,
      post_erase
    };

    callback_event (value v);
    operator value () const;
  };
}
  </pre>

  <p>The second argument to the callback function is the database
     on which the operation is about to be performed or has just
     been performed. A callback function can be inline or virtual.</p>

  <p>The callback function for the <code>*_persist</code>,
     <code>*_update</code>, and <code>*_erase</code> events is always
     called on the constant object reference while for the <code>*_load</code>
     events &mdash; always on the unrestricted reference.</p>

  <p>If only the non-<code>const</code> version of the callback function
     is provided, then only the <code>*_load</code> events will be delivered.
     If only the <code>const</code> version is provided, then all the
     events will be delivered to this function. Finally, if both versions
     are provided, then the <code>*_load</code> events will be delivered
     to the non-<code>const</code> version while all others &mdash; to the
     <code>const</code> version. If you need to modify the object in one
     of the "<code>const</code>" events, then you can safely cast away
     <code>const</code>-ness using the <code>const_cast</code> operator if
     you know that none of the objects will be created const. Alternatively,
     if you cannot make this assumption, then you can declare the data
     members you wish to modify as <code>mutable</code>.</p>

  <p>A database operations callback can be used to implement object-specific
     pre and post initializations, registrations, and cleanups. As an example,
     the following code fragment outlines an implementation of a
     <code>person</code> class that maintains the transient <code>age</code>
     data member in addition to the persistent date of birth. A callback
     is used to calculate the value of the former from the latter every
     time a <code>person</code> object is loaded from the database.</p>

  <pre class="cxx">
#include &lt;odb/core.hxx>
#include &lt;odb/callback.hxx>

#pragma db object callback(init)
class person
{
  ...

private:
  friend class odb::access;

  date born_;

  #pragma db transient
  unsigned short age_;

  void
  init (odb::callback_event e, odb::database&amp;)
  {
    switch (e)
    {
    case odb::callback_event::post_load:
    {
      // Calculate age from the date of birth.
      ...
      break;
    }
    default:
      break;
    }
  }
};
 </pre>

  <h3><a name="14.1.8">14.1.8 <code>schema</code></a></h3>

  <p>The <code>schema</code> specifier specifies a database schema
     that should be used for the persistent class.</p>

  <p>In relational databases the term schema can refer to two related
     but ultimately different concepts. Normally it means a collection
     of tables, indexes, sequences, etc., that are created in the
     database or the actual DDL statements that create these
     database objects. Some database implementations support what
     would be more accurately called a <em>database namespace</em>
     but is also called a schema. In this sense, a schema is a
     separate namespace in which tables, indexes, sequences, etc.,
     can be created. For example, two tables that have the same
     name can coexist in the same database if they belong to
     different schemas. In this section when we talk about a
     schema, we refer to the <em>database namespace</em> meaning
     of this term. </p>

  <p>When schemas are in use, a database object name is qualified
     with a schema. For example:</p>

  <pre class="sql">
CREATE TABLE accounting.employee (...)

SELECT ... FROM accounting.employee WHERE ...
  </pre>

  <p>In the above example <code>accounting</code> is the schema
     and the <code>employee</code> table belongs to this
     schema.</p>

  <p>Not all database implementations support schemas. Some
     implementation that don't support schemas (for example,
     MySQL, SQLite) allow the use of the above syntax to specify
     the database name. Yet others may support several levels
     of qualification. For example, Microsoft SQL Server has
     three levels starting with the linked database server,
     followed by the database, and then followed by
     the schema:
     <code>server1.company1.accounting.employee</code>.
     While the actual meaning of the qualifier in a qualified name
     vary from one database implementation to another, here we
     refer to all of them collectively as a schema.</p>

  <p>In ODB, a schema for a table of a persistent class can be
     specified at the class level, C++ namespace level, or the
     file level. To assign a schema to a specific persistent class
     we can use the <code>schema</code> specifier, for example:</p>

  <pre class="cxx">
#pragma db object schema("accounting")
class employee
{
  ...
};
  </pre>

  <p>If we are also assigning a table name, then we can use
     a shorter notation by specifying both the schema and
     the table name in the <code>table</code> specifier:</p>

  <pre class="cxx">
#pragma db object table("accounting.employee")
class employee
{
  ...
};
  </pre>

  <p>If we want to assign a schema to all the persistent classes
     in a C++ namespace, then, instead of specifying the schema
     for each class, we can specify it once at the C++ namespace level.
     For example:</p>

  <pre class="cxx">
#pragma db namespace schema("accounting")
namespace accounting
{
  #pragma db object
  class employee
  {
    ...
  };

  #pragma db object
  class employer
  {
    ...
  };
}
  </pre>

  <p>If we want to assign a schema to all the persistent classes in
     a file, then we can use the <code>--schema</code> ODB compiler
     option. For example:</p>

  <pre class="terminal">
odb ... --schema accounting ...
  </pre>

  <p>An alternative to this approach with the same effect is to
     assign a schema to the global namespace:</p>

  <pre class="cxx">
#pragma db namespace() schema("accounting")
  </pre>

  <p>By default schema qualifications are accumulated starting from
     the persistent class, continuing with the namespace hierarchy
     to which this class belongs, and finishing with the schema
     specified with the <code>--schema</code> option. For
     example:</p>

  <pre class="cxx">
#pragma db namespace schema("audit_db")
namespace audit
{
  #pragma db namespace schema("accounting")
  namespace accounting
  {
    #pragma db object
    class employee
    {
      ...
    };
  }
}
  </pre>

  <p>If we compile the above code fragment with the
     <code>--schema&nbsp;server1</code> option, then the
     <code>employee</code> table will have the
     <code>server1.audit_db.accounting.employee</code> qualified
     name.</p>

  <p>In some situations we may want to prevent such accumulation
     of the qualifications. To accomplish this we can use the
     so-called fully-qualified names, which have the empty leading
     name component. This is analogous to the C++ fully-qualified
     names in the <code>::accounting::employee</code> form. For
     example:</p>

  <pre class="cxx">
#pragma db namespace schema("accounting")
namespace accounting
{
  #pragma db object schema(".hr")
  class employee
  {
    ...
  };

  #pragma db object
  class employer
  {
    ...
  };
}
  </pre>

  <p>In the above code fragment, the <code>employee</code> table will
     have the <code>hr.employee</code> qualified name while the
     <code>employer</code> &mdash; <code>accounting.employer</code>.
     Note also that the empty leading name component is a special
     ODB syntax and is not propagated to the actual database names
     (using a name like <code>.hr.employee</code> to refer to a table
     will most likely result in an error).</p>

  <p>Auxiliary database objects for a persistent class, such as indexes,
     sequences, triggers, etc., are all created in the same schema
     as the class table. By default, this is also true for the
     container tables. However, if you need to store a container
     table in a different schema, then you can provide a qualified
     name using the <code>table</code> specifier, for example:</p>

  <pre class="cxx">
#pragma db object table("accounting.employee")
class employee
{
  ...

  #pragma db object table("operations.projects")
  std::vector&lt;std::string> projects_;
};
  </pre>

  <p>The standard syntax for qualified names used in the
     <code>schema</code> and <code>table</code> specifiers as well
     as the view <code>column</code> specifier (<a href="#14.4.10">Section
     14.4.10, "<code>column</code> (view)"</a>) has the
     <code>"</code><i>name</i><code>.</code><i>name</i>...<code>"</code>
     form where, as discussed above, the leading name component
     can be empty to denote a fully qualified name. This form, however,
     doesn't work if one of the name components contains periods. To
     support such cases the alternative form is available:
     <code>"</code><i>name</i><code>"."</code><i>name</i><code>"</code>...
     For example:</p>

  <pre class="cxx">
#pragma db object table("accounting_1.2"."employee")
class employee
{
  ...
};
  </pre>

  <p>Finally, to specify an unqualified name that contains periods
     we can use the following special syntax:</p>

  <pre class="cxx">
#pragma db object schema(."accounting_1.2") table("employee")
class employee
{
  ...
};
  </pre>

  <p>Table prefixes (<a href="#14.5.2">Section 14.5.2, "<code>table</code>"</a>)
     can be used as an alternative to database schemas if the target
     database system does not support schemas.</p>

  <h3><a name="14.1.9">14.1.9 <code>polymorphic</code></a></h3>

  <p>The <code>polymorphic</code> specifier specifies that the persistent
     class is polymorphic. For more information on polymorphism support,
     refer to <a href="#8">Chapter 8, "Inheritance"</a>.</p>

  <h3><a name="14.1.10">14.1.10 <code>session</code></a></h3>

  <p>The <code>session</code> specifier specifies whether to enable
     session support for the persistent class. For example:</p>

  <pre class="cxx">
#pragma db object session        // Enable.
class person
{
  ...
};

#pragma db object session(true)  // Enable.
class employee
{
  ...
};

#pragma db object session(false) // Disable.
class employer
{
  ...
};
  </pre>

  <p>Session support is disabled by default unless the
     <code>--generate-session</code> ODB compiler option is specified
     or session support is enabled at the namespace level
     (<a href="#14.5.4">Section 14.5.4, "<code>session</code>"</a>).
     For more information on sessions, refer to <a href="#11">Chapter
     11, "Session"</a>.</p>

  <h3><a name="14.1.11">14.1.11 <code>definition</code></a></h3>

  <p>The <code>definition</code> specifier specifies an alternative
     <em>definition location</em> for the persistent class. By
     default, the ODB compiler generates the database support code for
     a persistent class when we compile the header file that
     defines this class. However, if the  <code>definition</code>
     specifier is used, then the ODB compiler will instead generate
     the database support code when we compile the header file
     containing this pragma.</p>

  <p>For more information on this functionality, refer to
     <a href="#14.3.7">Section 14.3.7, "<code>definition</code>"</a>.</p>

  <h3><a name="14.1.12">14.1.12 <code>transient</code></a></h3>

  <p>The <code>transient</code> specifier instructs the ODB compiler to
     treat all non-virtual data members in the persistent class as transient
     (<a href="#14.4.1">Section 14.4.1, "<code>transient</code>"</a>).
     This specifier is primarily useful when declaring virtual data
     members, as discussed in <a href="#14.4.13">Section 14.4.13,
     "<code>virtual</code>"</a>.</p>

  <h3><a name="14.1.13">14.1.13 <code>sectionable</code></a></h3>

  <p>The <code>sectionable</code> specifier instructs the ODB compiler
     to generate support for the addition of new object sections in
     derived classes in a hierarchy with the optimistic concurrency
     model. For more information on this functionality, refer to
     <a href="#9.2">Section 9.2, "Sections and Optimistic
     Concurrency"</a>.</p>

  <h3><a name="14.1.14">14.1.14 <code>deleted</code></a></h3>

  <p>The <code>deleted</code> specifier marks the persistent class as
     soft-deleted. The single required argument to this specifier is
     the deletion version. For more information on this functionality,
     refer to <a href="#13.4">Section 13.4, "Soft Object Model
     Changes"</a>.</p>

  <h3><a name="14.1.15">14.1.15 <code>bulk</code></a></h3>

  <p>The <code>bulk</code> specifier enables bulk operation support for
     the persistent class. The single required argument to this specifier
     is the batch size. For more information on this functionality, refer
     to <a href="#15.3">Section 15.3, "Bulk Database Operations"</a>.</p>

  <h3><a name="14.1.16">14.1.16 <code>options</code></a></h3>

    <p>The <code>options</code> specifier specifies additional table
       definition options that should be used for the persistent class. For
       example:</p>

  <pre class="cxx">
#pragma db object options("PARTITION BY RANGE (age)")
class person
{
  ...

  unsigned short age_;
};
  </pre>

    <p>Table definition options for a container table can be specified with
       the <code>options</code> data member specifier
       (<a href="#14.4.8">Section 14.4.8, "<code>options</code>"</a>). For
       example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db options("PARTITION BY RANGE (index)")
  std::vector&lt;std::string> aliases_;
};
  </pre>


  <h2><a name="14.2">14.2 View Type Pragmas</a></h2>

  <p>A pragma with the <code>view</code> qualifier declares a C++ class
     as a view type. The qualifier can be optionally followed,
     in any order, by one or more specifiers summarized in the
     table below:</p>

  <!-- border="1" is necessary for html2ps -->
  <table class="specifiers" border="1">
    <tr>
      <th>Specifier</th>
      <th>Summary</th>
      <th>Section</th>
    </tr>

    <tr>
      <td><code>object</code></td>
      <td>object associated with a view</td>
      <td><a href="#14.2.1">14.2.1</a></td>
    </tr>

    <tr>
      <td><code>table</code></td>
      <td>table associated with a view</td>
      <td><a href="#14.2.2">14.2.2</a></td>
    </tr>

    <tr>
      <td><code>query</code></td>
      <td>view query condition</td>
      <td><a href="#14.2.3">14.2.3</a></td>
    </tr>

    <tr>
      <td><code>pointer</code></td>
      <td>pointer type for a view</td>
      <td><a href="#14.2.4">14.2.4</a></td>
    </tr>

    <tr>
      <td><code>callback</code></td>
      <td>database operations callback</td>
      <td><a href="#14.2.5">14.2.5</a></td>
    </tr>

    <tr>
      <td><code>definition</code></td>
      <td>definition location for a view</td>
      <td><a href="#14.2.6">14.2.6</a></td>
    </tr>

    <tr>
      <td><code>transient</code></td>
      <td>all non-virtual data members in a view are transient</td>
      <td><a href="#14.2.7">14.2.7</a></td>
    </tr>

  </table>

  <p>For more information on view types refer to <a href="#10"> Chapter 10,
     "Views"</a>.</p>

  <h3><a name="14.2.1">14.2.1 <code>object</code></a></h3>

  <p>The <code>object</code> specifier specifies a persistent class
     that should be associated with the view. For more information
     on object associations refer to <a href="#10.1">Section 10.1, "Object
     Views"</a>.</p>

  <h3><a name="14.2.2">14.2.2 <code>table</code></a></h3>

  <p>The <code>table</code> specifier specifies a database table
     that should be associated with the view. For more information
     on table associations refer to <a href="#10.3">Section 10.3, "Table
     Views"</a>.</p>

  <h3><a name="14.2.3">14.2.3 <code>query</code></a></h3>

  <p>The <code>query</code> specifier specifies a query condition
     and, optionally, result modifiers for an object or table view
     or a native SQL query for a native view. An empty <code>query</code>
     specifier indicates that a native SQL query is provided at runtime.
     For more information on query conditions refer to
     <a href="#10.5">Section 10.5, "View Query Conditions"</a>. For
     more information on native SQL queries, refer to
     <a href="#10.6">Section 10.6, "Native Views"</a>.</p>

  <h3><a name="14.2.4">14.2.4 <code>pointer</code></a></h3>

  <p>The <code>pointer</code> specifier specifies the view pointer type
     for the view class. Similar to objects, the view pointer type is used
     to return dynamically allocated instances of a view class. The
     semantics of the <code>pointer</code> specifier for a view are the
     same as those of the <code>pointer</code> specifier for an object
     (<a href="#14.1.2">Section 14.1.2, "<code>pointer</code>"</a>).</p>

  <h3><a name="14.2.5">14.2.5 <code>callback</code></a></h3>

  <p>The <code>callback</code> specifier specifies the view class
     member function that should be called before and after an
     instance of this view class is created as part of the query
     result iteration. The semantics of the <code>callback</code>
     specifier for a view are similar to those of the
     <code>callback</code> specifier for an object
     (<a href="#14.1.7">Section 14.1.7, "<code>callback</code>"</a>)
     except that the only events that can trigger a callback
     call in the case of a view are <code>pre_load</code> and
     <code>post_load</code>.</p>

  <h3><a name="14.2.6">14.2.6 <code>definition</code></a></h3>

  <p>The <code>definition</code> specifier specifies an alternative
     <em>definition location</em> for the view class. By
     default, the ODB compiler generates the database support code for
     a view class when we compile the header file that
     defines this class. However, if the  <code>definition</code>
     specifier is used, then the ODB compiler will instead generate
     the database support code when we compile the header file
     containing this pragma.</p>

  <p>For more information on this functionality, refer to
     <a href="#14.3.7">Section 14.3.7, "<code>definition</code>"</a>.</p>

  <h3><a name="14.2.7">14.2.7 <code>transient</code></a></h3>

  <p>The <code>transient</code> specifier instructs the ODB compiler
     to treat all non-virtual data members in the view class as transient
     (<a href="#14.4.1">Section 14.4.1, "<code>transient</code>"</a>).
     This specifier is primarily useful when declaring virtual data
     members, as discussed in <a href="#14.4.13">Section 14.4.13,
     "<code>virtual</code>"</a>.</p>

  <h2><a name="14.3">14.3 Value Type Pragmas</a></h2>

  <p>A pragma with the <code>value</code> qualifier describes a value
     type. It can be optionally followed, in any order, by one or more
     specifiers summarized in the table below:</p>

  <!-- border="1" is necessary for html2ps -->
  <table class="specifiers" border="1">
    <tr>
      <th>Specifier</th>
      <th>Summary</th>
      <th>Section</th>
    </tr>

    <tr>
      <td><code>type</code></td>
      <td>database type for a value type</td>
      <td><a href="#14.3.1">14.3.1</a></td>
    </tr>

    <tr>
      <td><code>id_type</code></td>
      <td>database type for a value type when used as an object id</td>
      <td><a href="#14.3.2">14.3.2</a></td>
    </tr>

    <tr>
      <td><code>null</code>/<code>not_null</code></td>
      <td>type can/cannot be <code>NULL</code></td>
      <td><a href="#14.3.3">14.3.3</a></td>
    </tr>

    <tr>
      <td><code>default</code></td>
      <td>default value for a value type</td>
      <td><a href="#14.3.4">14.3.4</a></td>
    </tr>

    <tr>
      <td><code>options</code></td>
      <td>database options for a value type</td>
      <td><a href="#14.3.5">14.3.5</a></td>
    </tr>

    <tr>
      <td><code>readonly</code></td>
      <td>composite value type is read-only</td>
      <td><a href="#14.3.6">14.3.6</a></td>
    </tr>

    <tr>
      <td><code>definition</code></td>
      <td>definition location for a composite value type</td>
      <td><a href="#14.3.7">14.3.7</a></td>
    </tr>

    <tr>
      <td><code>transient</code></td>
      <td>all non-virtual data members in a composite value are transient</td>
      <td><a href="#14.3.8">14.3.8</a></td>
    </tr>

    <tr>
      <td><code>unordered</code></td>
      <td>ordered container should be stored unordered</td>
      <td><a href="#14.3.9">14.3.9</a></td>
    </tr>

    <tr>
      <td><code>index_type</code></td>
      <td>database type for a container's index type</td>
      <td><a href="#14.3.10">14.3.10</a></td>
    </tr>

    <tr>
      <td><code>key_type</code></td>
      <td>database type for a container's key type</td>
      <td><a href="#14.3.11">14.3.11</a></td>
    </tr>

    <tr>
      <td><code>value_type</code></td>
      <td>database type for a container's value type</td>
      <td><a href="#14.3.12">14.3.12</a></td>
    </tr>

    <tr>
      <td><code>value_null</code>/<code>value_not_null</code></td>
      <td>container's value can/cannot be <code>NULL</code></td>
      <td><a href="#14.3.13">14.3.13</a></td>
    </tr>

    <tr>
      <td><code>id_options</code></td>
      <td>database options for a container's id column</td>
      <td><a href="#14.3.14">14.3.14</a></td>
    </tr>

    <tr>
      <td><code>index_options</code></td>
      <td>database options for a container's index column</td>
      <td><a href="#14.3.15">14.3.15</a></td>
    </tr>

    <tr>
      <td><code>key_options</code></td>
      <td>database options for a container's key column</td>
      <td><a href="#14.3.16">14.3.16</a></td>
    </tr>

    <tr>
      <td><code>value_options</code></td>
      <td>database options for a container's value column</td>
      <td><a href="#14.3.17">14.3.17</a></td>
    </tr>

    <tr>
      <td><code>id_column</code></td>
      <td>column name for a container's object id</td>
      <td><a href="#14.3.18">14.3.18</a></td>
    </tr>

    <tr>
      <td><code>index_column</code></td>
      <td>column name for a container's index</td>
      <td><a href="#14.3.19">14.3.19</a></td>
    </tr>

    <tr>
      <td><code>key_column</code></td>
      <td>column name for a container's key</td>
      <td><a href="#14.3.20">14.3.20</a></td>
    </tr>

    <tr>
      <td><code>value_column</code></td>
      <td>column name for a container's value</td>
      <td><a href="#14.3.21">14.3.21</a></td>
    </tr>

  </table>

  <p>Many of the value type specifiers have corresponding member type
     specifiers with the same names (<a href="#14.4">Section 14.4,
     "Data Member Pragmas"</a>). The behavior of such specifiers
     for members is similar to that for value types. The only difference
     is the scope. A particular value type specifier applies to all the
     members of this value type that don't have a pre-member version
     of the specifier, while the member specifier always applies only
     to a single member. Also, with a few exceptions, member specifiers
     take precedence over and override parameters specified with value
     specifiers.</p>

  <h3><a name="14.3.1">14.3.1 <code>type</code></a></h3>

  <p>The <code>type</code> specifier specifies the native database type
     that should be used for data members of this type. For example:</p>

  <pre class="cxx">
#pragma db value(bool) type("INT")

#pragma db object
class person
{
  ...

  bool married_; // Mapped to INT NOT NULL database type.
};
  </pre>

  <p>The ODB compiler provides the default mapping between common C++
     types, such as <code>bool</code>, <code>int</code>, and
     <code>std::string</code> and the database types for each supported
     database system. For more information on the default mapping,
     refer to <a href="#II">Part II, "Database Systems"</a>. The
     <code>null</code> and <code>not_null</code> (<a href="#14.3.3">Section
     14.3.3, "<code>null</code>/<code>not_null</code>"</a>) specifiers
     can be used to control the <code>NULL</code> semantics of a type.</p>

  <p>In the above example we changed the mapping for the <code>bool</code>
     type which is now mapped to the <code>INT</code> database type. In
     this case, the <code>value</code> pragma is all that is necessary
     since the ODB compiler will be able to figure out how to store
     a boolean value as an integer in the database. However, there
     could be situations where the ODB compiler will not know how to
     handle the conversion between the C++ and database representations
     of a value. Consider, as an example, a situation where the
     boolean value is stored in the database as a string:</p>

  <pre class="cxx">
#pragma db value(bool) type("VARCHAR(5)")
  </pre>

  <p>The possible database values for the C++ <code>true</code> value could
     be <code>"true"</code>, or <code>"TRUE"</code>, or <code>"True"</code>.
     Or, maybe, all of the above could be valid. The ODB compiler has no way
     of knowing how your application wants to convert <code>bool</code>
     to a string and back. To support such custom value type mappings,
     ODB allows you to provide your own database conversion functions
     by specializing the <code>value_traits</code> class template. The
     <code>mapping</code> example in the <code>odb-examples</code>
     package shows how to do this for all the supported database systems.</p>

  <h3><a name="14.3.2">14.3.2 <code>id_type</code></a></h3>

  <p>The <code>id_type</code> specifier specifies the native database type
     that should be used for data members of this type that are designated as
     object identifiers (<a href="#14.4.1">Section 14.4.1,
     "<code>id</code>"</a>). In combination with the <code>type</code>
     specifier (<a href="#14.3.1">Section 14.3.1, "<code>type</code>"</a>)
     <code>id_type</code> allows you to map a C++ type differently depending
     on whether it is used in an ordinary member or an object id. For
     example:</p>

  <pre class="cxx">
#pragma db value(std::string) type("TEXT") id_type("VARCHAR(64)")

#pragma db object
class person
{
  ...

  #pragma db id
  std::string email_; // Mapped to VARCHAR(64) NOT NULL.

  std::string name_;  // Mapped to TEXT NOT NULL.
};
  </pre>

  <p>Note that there is no corresponding member type specifier for
     <code>id_type</code> since the desired result can be achieved
     with just the <code>type</code> specifier, for example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db id type("VARCHAR(128)")
  std::string email_;
};
  </pre>

  <h3><a name="14.3.3">14.3.3 <code>null</code>/<code>not_null</code></a></h3>

  <p>The <code>null</code> and <code>not_null</code> specifiers specify that
     a value type or object pointer can or cannot be <code>NULL</code>,
     respectively. By default, value types are assumed not to allow
     <code>NULL</code> values while object pointers are assumed to
     allow <code>NULL</code> values. Data members of types that allow
     <code>NULL</code> values are mapped in a relational database to
     columns that allow <code>NULL</code> values. For example:</p>

  <pre class="cxx">
using string_ptr = std::shared_ptr&lt;std::string>;
#pragma db value(string_ptr) type("TEXT") null

#pragma db object
class person
{
  ...

  string_ptr name_; // Mapped to TEXT NULL.
};

using person_ptr = std::shared_ptr&lt;person>;
#pragma db value(person_ptr) not_null
  </pre>

  <p>The <code>NULL</code> semantics can also be specified on the
     per-member basis (<a href="#14.4.6">Section 14.4.6,
     "<code>null</code>/<code>not_null</code>"</a>). If both a type and
     a member have <code>null</code>/<code>not_null</code> specifiers,
     then the member specifier takes precedence. If a member specifier
     relaxes the <code>NULL</code> semantics (that is, if a member has
     the <code>null</code> specifier and the type has the explicit
     <code>not_null</code> specifier), then a warning is issued.</p>

  <p>It is also possible to override a previously specified
     <code>null</code>/<code>not_null</code> specifier. This is
     primarily useful if a third-party type, for example,
     one provided by a profile library (<a href="#III">Part III,
     "Profiles"</a>), allows <code>NULL</code> values but in your
     object model data members of this type should never be
     <code>NULL</code>. In this case you can use the <code>not_null</code>
     specifier to disable <code>NULL</code> values for this type for the
     entire translation unit. For example:</p>

  <pre class="cxx">
// By default, null_string allows NULL values.
//
#include &lt;null-string.hxx>

// Disable NULL values for all the null_string data members.
//
#pragma db value(null_string) not_null
  </pre>

  <p>For a more detailed discussion of the <code>NULL</code> semantics
     for values, refer to <a href="#7.3">Section 7.3, "Pointers and
     <code>NULL</code> Value Semantics"</a>. For a more detailed
     discussion of the <code>NULL</code> semantics for object pointers,
     refer to <a href="#6">Chapter 6, "Relationships"</a>.</p>

  <h3><a name="14.3.4">14.3.4 <code>default</code></a></h3>

  <p>The <code>default</code> specifier specifies the database default value
     that should be used for data members of this type. For example:</p>

  <pre class="cxx">
#pragma db value(std::string) default("")

#pragma db object
class person
{
  ...

  std::string name_; // Mapped to TEXT NOT NULL DEFAULT ''.
};
  </pre>

  <p>The semantics of the <code>default</code> specifier for a value type
     are similar to those of the <code>default</code> specifier for a
     data member (<a href="#14.4.7">Section 14.4.7,
     "<code>default</code>"</a>).</p>

  <h3><a name="14.3.5">14.3.5 <code>options</code></a></h3>

  <p>The <code>options</code> specifier specifies additional column
     definition options that should be used for data members of this
     type. For example:</p>

  <pre class="cxx">
#pragma db value(std::string) options("COLLATE binary")

#pragma db object
class person
{
  ...

  std::string name_; // Mapped to TEXT NOT NULL COLLATE binary.
};
  </pre>

  <p>The semantics of the <code>options</code> specifier for a value type
     are similar to those of the <code>options</code> specifier for a
     data member (<a href="#14.4.8">Section 14.4.8,
     "<code>options</code>"</a>).</p>

  <h3><a name="14.3.6">14.3.6 <code>readonly</code></a></h3>

  <p>The <code>readonly</code> specifier specifies that the composite
     value type is read-only. Changes to data members of a read-only
     composite value type are ignored when updating the database
     state of an object (<a href="#3.10">Section 3.10, "Updating Persistent
     Objects"</a>) containing such a value type. Note that this specifier
     is only valid for composite value types. For example:</p>

  <pre class="cxx">
#pragma db value readonly
class person_name
{
  ...
};
  </pre>

  <p>Read-only and read-write composite values can derive from each other
     without any restrictions. When a read-only value derives from a
     read-write value, the resulting whole value is read-only, including
     the part corresponding to the read-write base. On the other hand, when a
     read-write value derives from a read-only value, all the data
     members that correspond to the read-only base are treated as
     read-only while the rest is treated as read-write.</p>

  <p>Note that it is also possible to declare individual data members
     (<a href="#14.4.12">Section 14.4.12, "<code>readonly</code>"</a>)
     as well as whole objects (<a href="#14.1.4">Section 14.1.4,
     "<code>readonly</code>"</a>) as read-only.</p>

  <h3><a name="14.3.7">14.3.7 <code>definition</code></a></h3>

  <p>The <code>definition</code> specifier specifies an alternative
     <em>definition location</em> for the composite value type. By
     default, the ODB compiler generates the database support code for
     a composite value type when we compile the header file that
     defines this value type. However, if the  <code>definition</code>
     specifier is used, then the ODB compiler will instead generate
     the database support code when we compile the header file containing
     this pragma.</p>

  <p>This mechanism is primarily useful for converting third-party
     types to ODB composite value types. In such cases we normally
     cannot modify the header files to add the necessary pragmas.
     It is also often inconvenient to compile these header files
     with the ODB compiler. With the <code>definition</code>
     specifier we can create a <em>wrapper header</em> that contains
     the necessary pragmas and instructs the ODB compiler to generate
     the database support code for a third-party type when we compile
     the wrapper header. As an example, consider <code>struct timeval</code>
     that is defined in the <code>&lt;sys/time.h></code> system header.
     This type has the following (or similar) definition:</p>

  <pre class="cxx">
struct timeval
{
  long tv_sec;
  long tv_usec;
};
  </pre>

  <p>If we would like to make this type an ODB composite value type,
     then we can create a wrapper header, for example
     <code>time-mapping.hxx</code>, with the following content:</p>

  <pre class="cxx">
#ifndef TIME_MAPPING_HXX
#define TIME_MAPPING_HXX

#include &lt;sys/time.h>

#pragma db value(timeval) definition
#pragma db member(timeval::tv_sec) column("sec")
#pragma db member(timeval::tv_usec) column("usec")

#endif // TIME_MAPPING_HXX
  </pre>

  <p>If we now compile this header with the ODB compiler, the
     resulting <code>time-mapping-odb.?xx</code> files will
     contain the database support code for <code>struct timeval</code>.
     To use <code>timeval</code> in our persistent classes, we simply
     include the <code>time-mapping.hxx</code> header:</p>

  <pre class="cxx">
#include &lt;sys/time.h>
#include "time-mapping.hxx"

#pragma db object
class object
{
  timeval timestamp;
};
  </pre>

  <h3><a name="14.3.8">14.3.8 <code>transient</code></a></h3>

  <p>The <code>transient</code> specifier instructs the ODB compiler
     to treat all non-virtual data members in the composite value type
     as transient (<a href="#14.4.1">Section 14.4.1,
     "<code>transient</code>"</a>). This specifier is primarily useful
     when declaring virtual data members, as discussed in
     <a href="#14.4.13">Section 14.4.13, "<code>virtual</code>"</a>.</p>

  <h3><a name="14.3.9">14.3.9 <code>unordered</code></a></h3>

  <p>The <code>unordered</code> specifier specifies that the ordered
     container should be stored unordered in the database. The database
     table for such a container will not contain the index column
     and the order in which elements are retrieved from the database may
     not be the same as the order in which they were stored. For example:</p>

  <pre class="cxx">
using names = std::vector&lt;std::string>;
#pragma db value(names) unordered
  </pre>

  <p>For a more detailed discussion of ordered containers and their
     storage in the database, refer to <a href="#5.1">Section 5.1,
     "Ordered Containers"</a>.</p>

  <h3><a name="14.3.10">14.3.10 <code>index_type</code></a></h3>

  <p>The <code>index_type</code> specifier specifies the native
     database type that should be used for the ordered container's
     index column. The semantics of <code>index_type</code>
     are similar to those of the <code>type</code> specifier
     (<a href="#14.3.1">Section 14.3.1, "<code>type</code>"</a>). The native
     database type is expected to be an integer type. For example:</p>

  <pre class="cxx">
using names = std::vector&lt;std::string>;
#pragma db value(names) index_type("SMALLINT UNSIGNED")
  </pre>

  <h3><a name="14.3.11">14.3.11 <code>key_type</code></a></h3>

  <p>The <code>key_type</code> specifier specifies the native
     database type that should be used for the map container's
     key column. The semantics of <code>key_type</code>
     are similar to those of the <code>type</code> specifier
     (<a href="#14.3.1">Section 14.3.1, "<code>type</code>"</a>). For
     example:</p>

  <pre class="cxx">
using age_weight_map = std::map&lt;unsigned short, float>;
#pragma db value(age_weight_map) key_type("INT UNSIGNED")
  </pre>

  <h3><a name="14.3.12">14.3.12 <code>value_type</code></a></h3>

  <p>The <code>value_type</code> specifier specifies the native
     database type that should be used for the container's
     value column. The semantics of <code>value_type</code>
     are similar to those of the <code>type</code> specifier
     (<a href="#14.3.1">Section 14.3.1, "<code>type</code>"</a>). For
     example:</p>

  <pre class="cxx">
using names = std::vector&lt;std::string>;
#pragma db value(names) value_type("VARCHAR(255)")
  </pre>

  <p>The <code>value_null</code> and <code>value_not_null</code>
     (<a href="#14.3.13">Section 14.3.13,
     "<code>value_null</code>/<code>value_not_null</code>"</a>) specifiers
     can be used to control the <code>NULL</code> semantics of a value
     column.</p>

  <h3><a name="14.3.13">14.3.13 <code>value_null</code>/<code>value_not_null</code></a></h3>

  <p>The <code>value_null</code> and <code>value_not_null</code> specifiers
     specify that the container type's element value can or cannot be
     <code>NULL</code>, respectively. The semantics of <code>value_null</code>
     and <code>value_not_null</code> are similar to those of the
     <code>null</code> and <code>not_null</code> specifiers
     (<a href="#14.3.3">Section 14.3.3, "<code>null</code>/<code>not_null</code>"</a>).
     For example:</p>

  <pre class="cxx">
#pragma db object
class account
{
  ...
};

using accounts = std::vector&lt;std::shared_ptr&lt;account>>;
#pragma db value(accounts) value_not_null
  </pre>

  <p>For set and multiset containers (<a href="#5.2">Section 5.2, "Set and
     Multiset Containers"</a>) the element value is automatically treated
     as not allowing a <code>NULL</code> value.</p>


  <h3><a name="14.3.14">14.3.14 <code>id_options</code></a></h3>

  <p>The <code>id_options</code> specifier specifies additional
     column definition options that should be used for the container's
     id column. For example:</p>

  <pre class="cxx">
using nicknames = std::vector&lt;std::string>;
#pragma db value(nicknames) id_options("COLLATE binary")
  </pre>

  <p>The semantics of the <code>id_options</code> specifier for a container
     type are similar to those of the <code>id_options</code> specifier for
     a container data member (<a href="#14.4.29">Section 14.4.29,
     "<code>id_options</code>"</a>).</p>


  <h3><a name="14.3.15">14.3.15 <code>index_options</code></a></h3>

  <p>The <code>index_options</code> specifier specifies additional
     column definition options that should be used for the container's
     index column. For example:</p>

  <pre class="cxx">
using nicknames = std::vector&lt;std::string>;
#pragma db value(nicknames) index_options("ZEROFILL")
  </pre>

  <p>The semantics of the <code>index_options</code> specifier for a container
     type are similar to those of the <code>index_options</code> specifier for
     a container data member (<a href="#14.4.30">Section 14.4.30,
     "<code>index_options</code>"</a>).</p>


  <h3><a name="14.3.16">14.3.16 <code>key_options</code></a></h3>

  <p>The <code>key_options</code> specifier specifies additional
     column definition options that should be used for the container's
     key column. For example:</p>

  <pre class="cxx">
using properties = std::map&lt;std::string, std::string>;
#pragma db value(properties) key_options("COLLATE binary")
  </pre>

  <p>The semantics of the <code>key_options</code> specifier for a container
     type are similar to those of the <code>key_options</code> specifier for
     a container data member (<a href="#14.4.31">Section 14.4.31,
     "<code>key_options</code>"</a>).</p>


  <h3><a name="14.3.17">14.3.17 <code>value_options</code></a></h3>

  <p>The <code>value_options</code> specifier specifies additional
     column definition options that should be used for the container's
     value column. For example:</p>

  <pre class="cxx">
using nicknames = std::set&lt;std::string>;
#pragma db value(nicknames) value_options("COLLATE binary")
  </pre>

  <p>The semantics of the <code>value_options</code> specifier for a container
     type are similar to those of the <code>value_options</code> specifier for
     a container data member (<a href="#14.4.32">Section 14.4.32,
     "<code>value_options</code>"</a>).</p>


  <h3><a name="14.3.18">14.3.18 <code>id_column</code></a></h3>

  <p>The <code>id_column</code> specifier specifies the column
     name that should be used to store the object id in the
     container's table. For example:</p>

  <pre class="cxx">
using names = std::vector&lt;std::string>;
#pragma db value(names) id_column("id")
  </pre>

  <p>If the column name is not specified, then <code>object_id</code>
     is used by default.</p>

  <h3><a name="14.3.19">14.3.19 <code>index_column</code></a></h3>

  <p>The <code>index_column</code> specifier specifies the column
     name that should be used to store the element index in the
     ordered container's table. For example:</p>

  <pre class="cxx">
using names = std::vector&lt;std::string>;
#pragma db value(names) index_column("name_number")
  </pre>

  <p>If the column name is not specified, then <code>index</code>
     is used by default.</p>

  <h3><a name="14.3.20">14.3.20 <code>key_column</code></a></h3>

  <p>The <code>key_column</code> specifier specifies the column
     name that should be used to store the key in the map
     container's table. For example:</p>

  <pre class="cxx">
using age_weight_map = std::map&lt;unsigned short, float>;
#pragma db value(age_weight_map) key_column("age")
  </pre>

  <p>If the column name is not specified, then <code>key</code>
     is used by default.</p>

  <h3><a name="14.3.21">14.3.21 <code>value_column</code></a></h3>

  <p>The <code>value_column</code> specifier specifies the column
     name that should be used to store the element value in the
     container's table. For example:</p>

  <pre class="cxx">
using age_weight_map = std::map&lt;unsigned short, float>;
#pragma db value(age_weight_map) value_column("weight")
  </pre>

  <p>If the column name is not specified, then <code>value</code>
     is used by default.</p>

  <!-- Data Member Pragmas -->


  <h2><a name="14.4">14.4 Data Member Pragmas</a></h2>

  <p>A pragma with the <code>member</code> qualifier or a positioned
     pragma without a qualifier describes a data member. It can
     be optionally followed, in any order, by one or more specifiers
     summarized in the table below:</p>

  <!-- border="1" is necessary for html2ps -->
  <table class="specifiers" border="1">
    <tr>
      <th>Specifier</th>
      <th>Summary</th>
      <th>Section</th>
    </tr>

    <tr>
      <td><code>id</code></td>
      <td>member is an object id</td>
      <td><a href="#14.4.1">14.4.1</a></td>
    </tr>

    <tr>
      <td><code>auto</code></td>
      <td>id is assigned by the database</td>
      <td><a href="#14.4.2">14.4.2</a></td>
    </tr>

    <tr>
      <td><code>type</code></td>
      <td>database type for a member</td>
      <td><a href="#14.4.3">14.4.3</a></td>
    </tr>

    <tr>
      <td><code>id_type</code></td>
      <td>database type for a member when used as an object id</td>
      <td><a href="#14.4.4">14.4.4</a></td>
    </tr>

    <tr>
      <td><code>get</code>/<code>set</code>/<code>access</code></td>
      <td>member accessor/modifier expressions</td>
      <td><a href="#14.4.5">14.4.5</a></td>
    </tr>

    <tr>
      <td><code>null</code>/<code>not_null</code></td>
      <td>member can/cannot be <code>NULL</code></td>
      <td><a href="#14.4.6">14.4.6</a></td>
    </tr>

    <tr>
      <td><code>default</code></td>
      <td>default value for a member</td>
      <td><a href="#14.4.7">14.4.7</a></td>
    </tr>

    <tr>
      <td><code>options</code></td>
      <td>database options for a member</td>
      <td><a href="#14.4.8">14.4.8</a></td>
    </tr>

    <tr>
      <td><code>column</code></td>
      <td>column name for a member of an object or composite value</td>
      <td><a href="#14.4.9">14.4.9</a></td>
    </tr>

    <tr>
      <td><code>column</code></td>
      <td>column name for a member of a view</td>
      <td><a href="#14.4.10">14.4.10</a></td>
    </tr>

    <tr>
      <td><code>transient</code></td>
      <td>member is not stored in the database</td>
      <td><a href="#14.4.11">14.4.11</a></td>
    </tr>

    <tr>
      <td><code>readonly</code></td>
      <td>member is read-only</td>
      <td><a href="#14.4.12">14.4.12</a></td>
    </tr>

    <tr>
      <td><code>virtual</code></td>
      <td>declare a virtual data member</td>
      <td><a href="#14.4.13">14.4.13</a></td>
    </tr>

    <tr>
      <td><code>inverse</code></td>
      <td>member is an inverse side of a bidirectional relationship</td>
      <td><a href="#14.4.14">14.4.14</a></td>
    </tr>

    <tr>
      <td><code>on_delete</code></td>
      <td><code>ON DELETE</code> clause for object pointer member</td>
      <td><a href="#14.4.15">14.4.15</a></td>
    </tr>

    <tr>
      <td><code>version</code></td>
      <td>member stores object version</td>
      <td><a href="#14.4.16">14.4.16</a></td>
    </tr>

    <tr>
      <td><code>index</code></td>
      <td>define database index for a member</td>
      <td><a href="#14.4.17">14.4.17</a></td>
    </tr>

    <tr>
      <td><code>unique</code></td>
      <td>define unique database index for a member</td>
      <td><a href="#14.4.18">14.4.18</a></td>
    </tr>

    <tr>
      <td><code>unordered</code></td>
      <td>ordered container should be stored unordered</td>
      <td><a href="#14.4.19">14.4.19</a></td>
    </tr>

    <tr>
      <td><code>table</code></td>
      <td>table name for a container</td>
      <td><a href="#14.4.20">14.4.20</a></td>
    </tr>

    <tr>
      <td><code>load</code>/<code>update</code></td>
      <td>loading/updating behavior for a section</td>
      <td><a href="#14.4.21">14.4.21</a></td>
    </tr>

    <tr>
      <td><code>section</code></td>
      <td>member belongs to a section</td>
      <td><a href="#14.4.22">14.4.22</a></td>
    </tr>

    <tr>
      <td><code>added</code></td>
      <td>member is soft-added</td>
      <td><a href="#14.4.23">14.4.23</a></td>
    </tr>

    <tr>
      <td><code>deleted</code></td>
      <td>member is soft-deleted</td>
      <td><a href="#14.4.24">14.4.24</a></td>
    </tr>

    <tr>
      <td><code>index_type</code></td>
      <td>database type for a container's index type</td>
      <td><a href="#14.4.25">14.4.25</a></td>
    </tr>

    <tr>
      <td><code>key_type</code></td>
      <td>database type for a container's key type</td>
      <td><a href="#14.4.26">14.4.26</a></td>
    </tr>

    <tr>
      <td><code>value_type</code></td>
      <td>database type for a container's value type</td>
      <td><a href="#14.4.27">14.4.27</a></td>
    </tr>

    <tr>
      <td><code>value_null</code>/<code>value_not_null</code></td>
      <td>container's value can/cannot be <code>NULL</code></td>
      <td><a href="#14.4.28">14.4.28</a></td>
    </tr>

    <tr>
      <td><code>id_options</code></td>
      <td>database options for a container's id column</td>
      <td><a href="#14.4.29">14.4.29</a></td>
    </tr>

    <tr>
      <td><code>index_options</code></td>
      <td>database options for a container's index column</td>
      <td><a href="#14.4.30">14.4.30</a></td>
    </tr>

    <tr>
      <td><code>key_options</code></td>
      <td>database options for a container's key column</td>
      <td><a href="#14.4.31">14.4.31</a></td>
    </tr>

    <tr>
      <td><code>value_options</code></td>
      <td>database options for a container's value column</td>
      <td><a href="#14.4.32">14.4.32</a></td>
    </tr>

    <tr>
      <td><code>id_column</code></td>
      <td>column name for a container's object id</td>
      <td><a href="#14.4.33">14.4.33</a></td>
    </tr>

    <tr>
      <td><code>index_column</code></td>
      <td>column name for a container's index</td>
      <td><a href="#14.4.34">14.4.34</a></td>
    </tr>

    <tr>
      <td><code>key_column</code></td>
      <td>column name for a container's key</td>
      <td><a href="#14.4.35">14.4.35</a></td>
    </tr>

    <tr>
      <td><code>value_column</code></td>
      <td>column name for a container's value</td>
      <td><a href="#14.4.36">14.4.36</a></td>
    </tr>

    <tr>
      <td><code>points_to</code></td>
      <td>establish relationship without object pointer</td>
      <td><a href="#14.4.37">14.4.37</a></td>
    </tr>

  </table>

  <p>Many of the member specifiers have corresponding value type
     specifiers with the same names (<a href="#14.3">Section 14.3,
     "Value Type Pragmas"</a>). The behavior of such specifiers
     for members is similar to that for value types. The only difference
     is the scope. A particular value type specifier applies to all the
     members of this value type that don't have a pre-member version
     of the specifier, while the member specifier always applies only
     to a single member. Also, with a few exceptions, member specifiers
     take precedence over and override parameters specified with value
     specifiers.</p>

  <h3><a name="14.4.1">14.4.1 <code>id</code></a></h3>

  <p>The <code>id</code> specifier specifies that the data member contains
     the object id. In a relational database, an identifier member is
     mapped to a primary key. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db id
  std::string email_;
};
  </pre>

  <p>Normally, every persistent class has a data member designated as an
     object's identifier. However, it is possible to declare a
     persistent class without an id using the object <code>no_id</code>
     specifier (<a href="#14.1.6">Section 14.1.6, "<code>no_id</code>"</a>).</p>

  <p>Note also that the <code>id</code> specifier cannot be used for data
     members of composite value types or views.</p>

  <h3><a name="14.4.2">14.4.2 <code>auto</code></a></h3>

  <p>The <code>auto</code> specifier specifies that the object's identifier
     is automatically assigned by the database. Only a member that was
     designated as an object id can have this specifier. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db id auto
  unsigned long long id_;
};
  </pre>

  <p>Note that automatically-assigned object ids are not reused.  If you have
     a high object turnover (that is, objects are routinely made persistent
     and then erased), then care must be taken not to run out of object
     ids. In such situations, using a 64-bit integer as the identifier type is
     a safe choice.</p>

  <p>For additional information on the automatic identifier assignment,
     refer to <a href="#3.8">Section 3.8, "Making Objects Persistent"</a>.</p>

  <p>Note also that the <code>auto</code> specifier cannot be specified
     for data members of composite value types or views.</p>

  <h3><a name="14.4.3">14.4.3 <code>type</code></a></h3>

  <p>The <code>type</code> specifier specifies the native database type
     that should be used for the data member. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db type("INT")
  bool married_;
};
  </pre>

  <p>The <code>null</code> and <code>not_null</code> (<a href="#14.4.6">Section
     14.4.6, "<code>null</code>/<code>not_null</code>"</a>) specifiers
     can be used to control the <code>NULL</code> semantics of a data member.
     It is also possible to specify the database type on the per-type instead
     of the per-member basis using the value <code>type</code>
     specifier (<a href="#14.3.1">Section 14.3.1, "<code>type</code>"</a>).</p>

  <h3><a name="14.4.4">14.4.4 <code>id_type</code></a></h3>

  <p>The <code>id_type</code> specifier specifies the native database type
     that should be used for the data member when it is part of an
     object identifier. This specifier only makes sense when applied to
     a member of a composite value type that is used for both id and
     non-id members. For example:</p>

  <pre class="cxx">
#pragma db value
class name
{
  ...

  #pragma db type("VARCHAR(256)") id_type("VARCHAR(64)")
  std::string first_;

  #pragma db type("VARCHAR(256)") id_type("VARCHAR(64)")
  std::string last_;
};

#pragma db object
class person
{
  ...

  #pragma db id
  name name_;  // name_.first_, name_.last_ mapped to VARCHAR(64)

  name alias_; // alias_.first_, alias_.last_ mapped to VARCHAR(256)
};
  </pre>

  <h3><a name="14.4.5">14.4.5 <code>get</code>/<code>set</code>/<code>access</code></a></h3>

  <p>The <code>get</code> and <code>set</code> specifiers specify the
     data member accessor and modifier expressions, respectively. If
     provided, the generated database support code will use these
     expressions to access and modify the data member when performing
     database operations. The <code>access</code> specifier can be used
     as a shortcut to specify both the accessor and modifier if they
     happen to be the same.</p>

  <p>In its simplest form the accessor or modifier expression can be
     just a name. Such a name should resolve either to another data
     member of the same type or to a suitable accessor or modifier
     member function. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

public:
  const std::string&amp; name () const;
  void name (const std::string&amp;);
private:
  #pragma db access(name)
  std::string name_;
};
  </pre>

  <p>A suitable accessor function is a <code>const</code> member function
     that takes no arguments and whose return value can be implicitly
     converted to the <code>const</code> reference to the member type
     (<code>const&nbsp;std::string&amp;</code> in the example above).
     An accessor function that returns a <code>const</code> reference
     to the data member is called <em>by-reference accessor</em>.
     Otherwise, it is called <em>by-value accessor</em>.</p>

  <p>A suitable modifier function can be of two forms. It can be the
     so called <em>by-reference modifier</em> which is a member function
     that takes no arguments and returns a non-<code>const</code> reference
     to the data member (<code>std::string&amp;</code> in the example above).
     Alternatively, it can be the so called <em>by-value modifier</em> which
     is a member function taking a single argument &mdash; the new value
     &mdash; that can be implicitly initialized from a variable of the member
     type (<code>std::string</code> in the example above). The return value
     of a by-value modifier, if any, is ignored. If both by-reference and
     by-value modifiers are available, then ODB prefers the by-reference
     version since it is more efficient. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

public:
  std::string get_name () const;      // By-value accessor.
  std::string&amp; set_name ();           // By-reference modifier.
  void set_name (std::string const&amp;); // By-value modifier.
private:
  #pragma db get(get_name) \ // Uses by-value accessor.
             set(set_name)   // Uses by-reference modifier.
  std::string name_;
};
  </pre>

  <p>Note that in many cases it is not necessary to specify accessor and
     modifier functions explicitly since the ODB compiler will try to
     discover them automatically in case the data member will be inaccessible
     to the generated code. In particular, in both of the above examples
     the ODB compiler would have successfully discovered the necessary
     functions. For more information on this functionality, refer to
     <a href="#3.2">Section 3.2, "Declaring Persistent Objects and
     Values"</a>.</p>

  <p>Note also that by-value accessors and by-value modifiers cannot be
     used for certain data members in certain situations. These limitations
     are discussed in more detail later in this section.</p>

  <p>Accessor and modifier expressions can be more elaborate than simple
     names. An accessor expression is any C++ expression that can be
     used to initialize a <code>const</code> reference to the member
     type. Similar to accessor functions, which are just a special case
     of accessor expressions, an accessor expression that evaluates to a
     <code>const</code> reference to the data member is called
     <em>by-reference accessor expression</em>. Otherwise, it is
     called <em>by-value accessor expression</em>.</p>

  <p>Modifier expressions can also be of two forms: <em>by-reference
     modifier expression</em> and <em>by-value modifier expression</em>
     (again, modifier functions are just a special case of modifier
     expressions). A by-reference modifier expression is any C++
     expression that evaluates to the non-<code>const</code> reference
     to the member type. A by-value modifier expression can be a
     single or multiple (separated by semicolon) C++ statements
     with the effect of setting the new member value.</p>

  <p>There are three special placeholders that are recognized by the
     ODB compiler in accessor and modifier expressions. The first
     is the <code>this</code> keyword which denotes a reference
     (note: not a pointer) to the persistent object. In accessor
     expressions this reference is <code>const</code> while in
     modifier expressions it is non-<code>const</code>. If an
     expression does not contain the <code>this</code> placeholder,
     then the ODB compiler automatically prefixes it with the
     <code>this.</code> sequence.</p>

  <p>The second placeholder, the <code>(?)</code> sequence, is used
     to denote the new value in by-value modifier expressions. The
     ODB compiler replaces the question mark with the variable name,
     keeping the surrounding parenthesis.</p>

  <p>The third placeholder, the <code>(!)</code> sequence, is used to denote
     the database instance in the modifier expressions. The ODB compiler
     replaces the exclamation mark with the reference to the database,
     keeping the surrounding parenthesis. The database instance can, for
     example, be used to load an object pointer.</p>

  <p>The following example shows a few more interesting accessor and
     modifier expressions:</p>

  <pre class="cxx">
#pragma db value
struct point
{
  point (int, int);

  int x;
  int y;
};

#pragma db object
class person
{
  ...

  public:
    const char* name () const;
    void name (const char*);
  private:
    #pragma db get(std::string (this.name ())) \
               set(name ((?).c_str ())) // The same as this.name (...).
    std::string name_;

  public:
    const std::unique_ptr&lt;account>&amp; acc () const;
    void acc (std::unique_ptr&lt;account>);
  private:
    #pragma db set(acc (std::move (?)))
    std::unique_ptr&lt;account> acc_;

  public:
    int loc_x () const
    int loc_y () const
    void loc_x (int);
    void loc_y (int);
  private:
    #pragma db get(point (this.loc_x (), this.loc_y ()))    \
               set(this.loc_x ((?).x); this.loc_y ((?).y))
    point loc_;
};
  </pre>

  <p>When the data member is of an array type, then the terms "reference"
     and "member type" in the above discussion should be replaced with
     "pointer" and "array element type", respectively. That is, the accessor
     expression for an array member is any C++ expression that can be
     used to initialize a <code>const</code> pointer to the array
     element type, and so on. The following example shows common
     accessor and modifier signatures for array members:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  public:
    const char* id () const; // By-reference accessor.
    void id (const char*);   // By-value modifier.
  private:
    char id_[16];

  public:
    const char* pub_key () const; // By-reference accessor.
    char* pub_key ();             // By-reference modifier.
  private:
    char pub_key_[2048];
};
  </pre>

  <p>Accessor and modifier expressions can be used with data members
     of simple value, composite value, container, and object pointer
     types. They can be used for data members in persistent classes,
     composite value types, and views. There is also a mechanism
     related to accessors and modifiers called virtual data members
     and which is discussed in <a href="#14.4.13">Section 14.4.13,
     "<code>virtual</code>"</a>.</p>

  <p>There are, however, certain limitations when it comes to using
     by-value accessor and modifier expressions. First of all, if a
     by-value modifier is used, then the data member type should be
     default-constructible. Furthermore, a composite value type that
     has a container member cannot be modified with a by-value modifier.
     Only a by-reference modifier expression can be used. The ODB
     compiler will detect such cases and issue diagnostics. For
     example:</p>

  <pre class="cxx">
#pragma db value
struct name
{
  std::string first_;
  std::string last_;
  std::vector&lt;std::string> aliases_;
};

#pragma db object
class person
{
  ...

public:
  const name&amp; name () const;
  void name (const name&amp;);
private:
  #pragma db access(name) // Error: by-value modifier.
  name name_;
};
  </pre>

  <p>In certain database systems it is also not possible to use by-value
     accessor and modifier expression with certain database types.
     The ODB compiler is only able to detect such cases and issue diagnostics
     if you specified accessor/modifier function names as opposed to custom
     expressions. For more information on these database and type-specific
     limitations, refer to the "Limitations" sections in <a href="#II">Part
     II, "Database Systems"</a>.</p>

  <h3><a name="14.4.6">14.4.6 <code>null</code>/<code>not_null</code></a></h3>

  <p>The <code>null</code> and <code>not_null</code> specifiers specify that
     the data member can or cannot be <code>NULL</code>, respectively.
     By default, data members of basic value types for which database
     mapping is provided by the ODB compiler do not allow <code>NULL</code>
     values while data members of object pointers allow <code>NULL</code>
     values. Other value types, such as those provided by the profile
     libraries (<a href="#III">Part III, "Profiles"</a>), may or may
     not allow <code>NULL</code> values, depending on the semantics
     of each value type. Consult the relevant documentation to find
     out more about the <code>NULL</code> semantics for such value
     types. A data member containing the object id (<a href="#14.4.1">Section
     14.4.1, "<code>id</code>"</a>) is automatically treated as not
     allowing a <code>NULL</code> value. Data members that
     allow <code>NULL</code> values are mapped in a relational database
     to columns that allow <code>NULL</code> values. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db null
  std::string name_;
};

#pragma db object
class account
{
  ...

  #pragma db not_null
  std::shared_ptr&lt;person> holder_;
};
  </pre>

  <p>The <code>NULL</code> semantics can also be specified on the
     per-type basis (<a href="#14.3.3">Section 14.3.3,
     "<code>null</code>/<code>not_null</code>"</a>). If both a type and
     a member have <code>null</code>/<code>not_null</code> specifiers,
     then the member specifier takes precedence. If a member specifier
     relaxes the <code>NULL</code> semantics (that is, if a member has
     the <code>null</code> specifier and the type has the explicit
     <code>not_null</code> specifier), then a warning is issued.</p>

  <p>For a more detailed discussion of the <code>NULL</code> semantics
     for values, refer to <a href="#7.3">Section 7.3, "Pointers and
     <code>NULL</code> Value Semantics"</a>. For a more detailed
     discussion of the <code>NULL</code> semantics for object pointers,
     refer to <a href="#6">Chapter 6, "Relationships"</a>.</p>

  <h3><a name="14.4.7">14.4.7 <code>default</code></a></h3>

  <p>The <code>default</code> specifier specifies the database default value
     that should be used for the data member. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db default(-1)
  int age_;              // Mapped to INT NOT NULL DEFAULT -1.
};
  </pre>

  <p>A default value can be the special <code>null</code> keyword,
     a <code>bool</code> literal (<code>true</code> or <code>false</code>),
     an integer literal, a floating point literal, a string literal, or
     an enumerator name. If you need to specify a default value that is
     an expression, for example an SQL function call, then you can use
     the <code>options</code> specifier (<a href="#14.4.8">Section
     14.4.8, "<code>options</code>"</a>) instead. For example:</p>

  <pre class="cxx">
enum gender {male, female, undisclosed};

#pragma db object
class person
{
  ...

  #pragma db default(null)
  odb::nullable&lt;std::string> middle_; // DEFAULT NULL

  #pragma db default(false)
  bool married_;                      // DEFAULT 0/FALSE

  #pragma db default(0.0)
  float weight_;                      // DEFAULT 0.0

  #pragma db default("Mr")
  string title_;                      // DEFAULT 'Mr'

  #pragma db default(undisclosed)
  gender gender_;                     // DEFAULT 2/'undisclosed'

  #pragma db options("DEFAULT CURRENT_TIMESTAMP()")
  date timestamp_;                    // DEFAULT CURRENT_TIMESTAMP()
};
  </pre>

  <p>Default values specified as enumerators are only supported for
     members that are mapped to an <code>ENUM</code> or an integer
     type in the database, which is the case for the automatic
     mapping of C++ enums and enum classes to suitable database
     types as performed by the ODB compiler. If you have mapped
     a C++ enum or enum class to another database type, then you
     should use a literal corresponding to that type to specify
     the default value. For example:</p>

  <pre class="cxx">
enum gender {male, female, undisclosed};
#pragma db value(gender) type("VARCHAR(11)")

#pragma db object
class person
{
  ...

  #pragma db default("undisclosed")
  gender gender_;                   // DEFAULT 'undisclosed'
};
  </pre>

  <p>A default value can also be specified on the per-type basis
     (<a href="#14.3.4">Section 14.3.4, "<code>default</code>"</a>).
     An empty <code>default</code> specifier can be used to reset
     a default value that was previously specified on the per-type
     basis. For example:</p>

  <pre class="cxx">
#pragma db value(std::string) default("")

#pragma db object
class person
{
  ...

  #pragma db default()
  std::string name_;   // No default value.
};
  </pre>

  <p>A data member containing the object id (<a href="#14.4.1">Section
     14.4.1, "<code>id</code>"</a> ) is automatically treated as not
     having a default value even if its type specifies a default value.</p>

  <p>Note also that default values do not affect the generated C++ code
     in any way. In particular, no automatic initialization of data members
     with their default values is performed at any point. If you need such
     an initialization, you will need to implement it yourself, for example,
     in your persistent class constructors. The default values only
     affect the generated database schemas and, in the context of ODB,
     are primarily useful for schema evolution.</p>

  <p>Additionally, the <code>default</code> specifier cannot be specified
     for view data members.</p>

  <h3><a name="14.4.8">14.4.8 <code>options</code></a></h3>

  <p>The <code>options</code> specifier specifies additional column
     definition options that should be used for the data member. For
     example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db options("CHECK(email != '')")
  std::string email_; // Mapped to TEXT NOT NULL CHECK(email != '').
};
  </pre>

  <p>Note that if specified for the container member, then instead of the
     column definition options it specifies the table definition options for
     the container table (<a href="#14.1.16">Section 14.1.16,
     "<code>options</code>"</a>).</p>

  <p>Options can also be specified on the per-type basis
     (<a href="#14.3.5">Section 14.3.5, "<code>options</code>"</a>).
     By default, options are accumulating. That is, the ODB compiler
     first adds all the options specified for a value type followed
     by all the options specified for a data member. To clear the
     accumulated options at any point in this sequence you can use
     an empty <code>options</code> specifier. For example:</p>

  <pre class="cxx">
#pragma db value(std::string) options("COLLATE binary")

#pragma db object
class person
{
  ...

  std::string first_; // TEXT NOT NULL COLLATE binary

  #pragma db options("CHECK(last != '')")
  std::string last_;  // TEXT NOT NULL COLLATE binary CHECK(last != '')

  #pragma db options()
  std::string title_; // TEXT NOT NULL

  #pragma db options() options("CHECK(email != '')")
  std::string email_; // TEXT NOT NULL CHECK(email != '')
};
  </pre>

  <p>ODB provides dedicated specifiers for specifying column types
     (<a href="#14.4.3">Section 14.4.3, "<code>type</code>"</a>),
     <code>NULL</code> constraints (<a href="#14.4.6">Section 14.4.6,
     "<code>null</code>/<code>not_null</code>"</a>), and default
     values (<a href="#14.4.7">Section 14.4.7, "<code>default</code>"</a>).
     For ODB to function correctly these specifiers should always be
     used instead of the opaque <code>options</code> specifier for
     these components of a column definition.</p>

  <p>Note also that the <code>options</code> specifier cannot be specified
     for view data members.</p>

  <h3><a name="14.4.9">14.4.9 <code>column</code> (object, composite value)</a></h3>

  <p>The <code>column</code> specifier specifies the column name
     that should be used to store the data member of a persistent class
     or composite value type in a relational database. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db id column("person_id")
  unsigned long long id_;
};
  </pre>

  <p>For a member of a composite value type, the <code>column</code> specifier
     specifies the column name prefix. Refer to <a href="#7.2.2">Section 7.2.2,
     "Composite Value Column and Table Names"</a> for details.</p>

  <p>If the column name is not specified, it is derived from the member's
     so-called public name. A public member name is obtained by removing
     the common data member name decorations, such as leading and trailing
     underscores, the <code>m_</code> prefix, etc.</p>

  <h3><a name="14.4.10">14.4.10 <code>column</code> (view)</a></h3>

  <p>The <code>column</code> specifier can be used to specify the associated
     object data member, the potentially qualified column name, or the column
     expression for the data member of a view class. For more information,
     refer to <a href="#10.1">Section 10.1, "Object Views"</a> and
     <a href="#10.3">Section 10.3, "Table Views"</a>.</p>

  <h3><a name="14.4.11">14.4.11 <code>transient</code></a></h3>

  <p>The <code>transient</code> specifier instructs the ODB compiler
     not to store the data member in the database. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  date born_;

  #pragma db transient
  unsigned short age_; // Computed from born_.
};
  </pre>

  <p>This pragma is usually used on computed members, pointers and
     references that are only meaningful in the application's
     memory, as well as utility members such as mutexes, etc.</p>

  <h3><a name="14.4.12">14.4.12 <code>readonly</code></a></h3>

  <p>The <code>readonly</code> specifier specifies that the data member of
     an object or composite value type is read-only. Changes to a read-only
     data member are ignored when updating the database state of an object
     (<a href="#3.10">Section 3.10, "Updating Persistent Objects"</a>)
     containing such a member. Since views are read-only, it is not
     necessary to use this specifier for view data members. Object id
     (<a href="#14.4.1">Section 14.4.1, "<code>id</code>"</a>)
     and inverse (<a href="#14.4.14">Section 14.4.14,
     "<code>inverse</code>"</a>) data members are automatically treated
     as read-only and must not be explicitly declared as such. For
     example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db readonly
  date born_;
};
  </pre>

  <p>Besides simple value members, object pointer, container, and composite
     value members can also be declared read-only. A change of a pointed-to
     object is ignored when updating the state of a read-only object
     pointer. Similarly, any changes to the number or order of
     elements or to the element values themselves are ignored when
     updating the state of a read-only container. Finally, any changes
     to the members of a read-only composite value type are also ignored
     when updating the state of such a composite value.</p>

  <p>ODB automatically treats <code>const</code> data members as read-only.
     For example, the following <code>person</code> object is equivalent
     to the above declaration for the database persistence purposes:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  const date born_; // Automatically read-only.
};
  </pre>

  <p>When declaring an object pointer <code>const</code>, make sure to
     declare the pointer as <code>const</code> rather than (or in addition
     to) the object itself. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  const person* father_; // Read-write pointer to a read-only object.
  person* const mother_; // Read-only pointer to a read-write object.
};
  </pre>

  <p>Note that in case of a wrapper type (<a href="#7.3">Section 7.3,
     "Pointers and <code>NULL</code> Value Semantics"</a>), both the
     wrapper and the wrapped type must be <code>const</code> in
     order for the ODB compiler to automatically treat the data
     member as read-only. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  const std::unique_ptr&lt;const date> born_;
};
  </pre>

  <p>Read-only members are useful when dealing with
     asynchronous changes to the state of a data member in the
     database which should not be overwritten. In other cases,
     where the state of a data member never changes, declaring such a member
     read-only allows ODB to perform more efficient object updates.
     In such cases, however, it is conceptually more correct to
     declare such a data member as <code>const</code> rather than
     as read-only.</p>

  <p>Note that it is also possible to declare composite value types
     (<a href="#14.3.6">Section 14.3.6, "<code>readonly</code>"</a>)
     as well as whole objects (<a href="#14.1.4">Section 14.1.4,
     "<code>readonly</code>"</a>) as read-only.</p>

  <h3><a name="14.4.13">14.4.13 <code>virtual</code></a></h3>

  <p>The <code>virtual</code> specifier is used to declare a virtual
     data member in an object, view, or composite value type. A virtual
     data member is an <em>imaginary</em> data member that is only
     used for the purpose of database persistence. A virtual data
     member does not actually exist (that is, occupy space) in the
     C++ class. Note also that virtual data members have nothing to
     do with C++ virtual functions or virtual inheritance. Specifically,
     no virtual function call overhead is incurred when using virtual
     data members.</p>

  <p>To declare a virtual data member we must specify the data
     member name using the <code>member</code> specifier. We must
     also specify the data member type with the <code>virtual</code>
     specifier. Finally, the virtual data member declaration must
     also specify the accessor and modifier expressions, unless
     suitable accessor and modifier functions can automatically be
     found by the ODB compiler (<a href="#14.4.5">Section 14.4.5,
     "<code>get</code>/<code>set</code>/<code>access</code>"</a>).
     For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  // Transient real data member that actually stores the data.
  //
  #pragma db transient
  std::string name_;

  // Virtual data member.
  //
  #pragma db member(name) virtual(std::string) access(name_)
};
  </pre>

  <p>From the pragma language point of view, a virtual data member
     behaves exactly like a normal data member. Specifically, we
     can reference the virtual data member after it has been
     declared and use positioned pragmas before its declaration.
     For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db transient
  std::string name_;

  #pragma db access(name_)
  #pragma db member(name) virtual(std::string)
};

#pragma db member(person::name) column("person_name")
#pragma db index member(person::name)
  </pre>

  <p>We can also declare a virtual data member outside the class
     scope:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  std::string name_;
};

#pragma db member(person::name_) transient
#pragma db member(person::name) virtual(std::string) access(name_)
  </pre>

  <p>The order of data members determines the order of columns in the
     resulting table. The order of virtual data members in relation to other
     data members (virtual or not) is the order of declaration, with virtual
     data members declared outside of the class coming last. This order,
     however, can be adjusted with the <code>before</code>
     and <code>after</code> specifiers. One of these specifiers without a
     parameter places the virtual member at the beginning or at the end of the
     members list, respectively. Alternatively, we can specify the member
     (virtual or not) before/after which this virtual member should be
     placed. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db id auto
  unsigned long long id_;

  #pragma db member(first) virtual(std::string) before
  #pragma db member(last)  virtual(std::string) after(first)
};
  </pre>

  <p>The order of columns in the resulting table will be: <code>first</code>,
     <code>last</code>, <code>id</code>.</p>

  <p>While in the above examples using virtual data members doesn't
     seem to yield any benefits, this mechanism can be useful in a
     number of situations. As one example, consider the need to
     aggregate or dis-aggregate a data member:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db transient
  std::pair&lt;std::string, std::string> name_;

  #pragma db member(first) virtual(std::string) access(name_.first)
  #pragma db member(last) virtual(std::string) access(name_.second)
};
  </pre>

  <p>We can also use virtual data members to implement composite
     object ids that are spread over multiple data members:</p>

  <pre class="cxx">
#pragma db value
struct name
{
  name () {}
  name (std::string const&amp; f, std::string const&amp; l)
    : first (f), last(l) {}

  std::string first;
  std::string last;
};

#pragma db object
class person
{
  ...

  #pragma db transient
  std::string first_;

  #pragma db transient
  std::string last_;

  #pragma db member(name) virtual(name) id                       \
             get(::name (this.first_, this.last_))               \
             set(this.first_ = (?).first; this.last_ = (?).last)
};
  </pre>

  <p>Another common situation that calls for virtual data members is
     a class that uses the pimpl idiom. While the following code
     fragment outlines the idea, for details refer to the
     <code>pimpl</code> example in the <code>odb-examples</code>
     package.</p>

  <pre class="cxx">
#pragma db object
class person
{
public:
  std::string const&amp; name () const;
  void name (std::string const&amp;);

  unsigned short age () const;
  void age (unsigned short);

  ...

private:
  class impl;

  #pragma db transient
  impl* pimpl_;

  #pragma db member(name) virtual(std::string)   // Uses name().
  #pragma db member(age) virtual(unsigned short) // Uses age().
};
  </pre>

  <p>The above example also shows that names used for virtual data
     members (<code>name</code> and <code>age</code> in our case) can
     be the same as the names of accessor/modifier functions. The only
     names that virtual data members cannot clash with are those of
     other data members, virtual or real.</p>

  <p>A common pattern in the above examples is the need to
     declare the real data member that actually stores the
     data as transient. If all the real data members in a
     class are treated as transient, then we can use the
     class-level <code>transient</code> specifier
     (<a href="#14.1.12">Section 14.1.12, "<code>transient</code>
     (object)"</a>,
     <a href="#14.3.8">Section 14.3.8, "<code>transient</code>
     (composite value)"</a>,
     <a href="#14.2.7">Section 14.2.7, "<code>transient</code>
     (view)"</a>)
     instead of doing it for each individual member. For example: </p>

  <pre class="cxx">
#pragma db object transient
class person
{
  ...

  std::string first_; // Transient.
  std::string last_;  // Transient.

  #pragma db member(name) virtual(name) ...
};
  </pre>

  <p>The ability to treat all the real data members as transient
     becomes more important if we don't know the names of these
     data members. This is often the case when we are working
     with third-party types that document the accessor and
     modifier functions but not the names of their private data
     members. As an example, consider the <code>point</code> class
     defined in a third-party <code>&lt;point></code> header file:</p>

  <pre class="cxx">
class point
{
public:
  point ();
  point (int x, int y);

  int x () const;
  int y () const;

  void x (int);
  void y (int);

private:
  ...
};
  </pre>

 <p>To convert this class to an ODB composite value type we could
    create the <code>point-mapping.hxx</code> file with the following
    content:</p>

  <pre class="cxx">
#include &lt;point>

#pragma db value(point) transient definition
#pragma db member(point::x) virtual(int)
#pragma db member(point::y) virtual(int)
  </pre>

  <p>Virtual data members can be of simple value, composite value,
     container, or object pointer types. They can be used in persistent
     classes, composite value types, and views.</p>

  <h3><a name="14.4.14">14.4.14 <code>inverse</code></a></h3>

  <p>The <code>inverse</code> specifier specifies that the data member of
     an object pointer or a container of object pointers type is an
     inverse side of a bidirectional object relationship. The single
     required argument to this specifier is the corresponding data
     member name in the referenced object. For example:</p>

  <pre class="cxx">
class person;

#pragma db object pointer(std::shared_ptr)
class employer
{
  ...

  std::vector&lt;std::shared_ptr&lt;person>> employees_;
};

#pragma db object pointer(std::shared_ptr)
class person
{
  ...

  #pragma db inverse(employee_)
  std::weak_ptr&lt;employer> employer_;
};
  </pre>

  <p>An inverse member does not have a corresponding column or, in case
     of a container, table in the resulting database schema. Instead, the
     column or table from the referenced object is used to retrieve the
     relationship information. Only ordered and set containers can be used
     for inverse members. If an inverse member is of an ordered container
     type, it is automatically marked as unordered
     (<a href="#14.4.19">Section 14.4.19, "<code>unordered</code>"</a>).</p>

  <p>For a more detailed discussion of inverse members, refer to
     <a href="#6.2">Section 6.2, "Bidirectional Relationships"</a>.</p>

  <h3><a name="14.4.15">14.4.15 <code>on_delete</code></a></h3>

  <p>The <code>on_delete</code> specifier specifies the on-delete semantics
     for a data member of an object pointer or a container of object
     pointers type. The single required argument to this specifier must
     be either <code>cascade</code> or <code>set_null</code>.</p>

  <p>The <code>on_delete</code> specifier is translated directly to the
     corresponding <code>ON DELETE</code> SQL clause. That is, if
     <code>cascade</code> is specified, then when a pointed-to object
     is erased from the database, the database state of the pointing
     object is automatically erased as well. If <code>set_null</code> is
     specified, then when a pointed-to object is erased from the database,
     the database state of the pointing object is automatically updated
     to set the pointer column to <code>NULL</code>. For example:</p>

  <pre class="cxx">
#pragma db object
class employer
{
  ...

  #pragma db id auto
  unsigned long long id_;
};

#pragma db object
class person
{
  ...

  #pragma db on_delete(cascade)
  employer* employer_;
};

unsigned long long id;

{
  employer e;
  person p;
  p.employer_ = &amp;e;

  transaction t (db.begin ());

  id = db.persist (e);
  db.persist (p);

  t.commit ();
}

{
  transaction t (db.begin ());

  // Database state of the person object is erased as well.
  //
  db.erase&lt;employer> (id);

  t.commit ();
}
  </pre>


  <p>Note that this is a database-level functionality and care must be
     taken in order not to end up with inconsistent object states in the
     application's memory and database. The following example illustrates
     the kind of problems one may encounter:</p>

  <pre class="cxx">
#pragma db object
class employer
{
  ...
};

#pragma db object
class person
{
  ...

  #pragma db on_delete(set_null)
  employer* employer_;
};

employer e;
person p;
p.employer_ = &amp;e;

{
  transaction t (db.begin ());
  db.persist (e);
  db.persist (p);
  t.commit ();
}

{
  transaction t (db.begin ());

  // The employer column is set to NULL in the database but
  // not the p.employer_ data member in the application.
  //
  db.erase (e);
  t.commit ();
}

{
  transaction t (db.begin ());

  // Override the employer column with an invalid pointer.
  //
  db.update (p);

  t.commit ();
}
  </pre>

  <p>Note that even optimistic concurrency will not resolve such
     issues unless you are using database-level support for optimistic
     concurrency as well (for example, <code>ROWVERSION</code> in SQL
     Server).</p>

  <p>The <code>on_delete</code> specifier is only valid for non-inverse
     object pointer data members. If the <code>set_null</code> semantics
     is used, then the pointer must allow the <code>NULL</code> value.</p>

  <h3><a name="14.4.16">14.4.16 <code>version</code></a></h3>

  <p>The <code>version</code> specifier specifies that the data member stores
     the object version used to support optimistic concurrency.  If a class
     has a version data member, then it must also be declared as having the
     optimistic concurrency model using the <code>optimistic</code> pragma
     (<a href="#14.1.5">Section 14.1.5, "<code>optimistic</code>"</a>). For
     example:</p>

  <pre class="cxx">
#pragma db object optimistic
class person
{
  ...

  #pragma db version
  unsigned long long version_;
};
  </pre>

  <p>A version member must be of an integral C++ type and must map to an
     integer or similar database type. Note also that object versions are not
     reused. If you have a high update frequency, then care must be taken not
     to run out of versions. In such situations, using a 64-bit integer as the
     version type is a safe choice.</p>

  <p>For a more detailed discussion of optimistic concurrency, refer to
     <a href="#12">Chapter 12, "Optimistic Concurrency"</a>.</p>

  <h3><a name="14.4.17">14.4.17 <code>index</code></a></h3>

  <p>The <code>index</code> specifier instructs the ODB compiler to define
     a database index for the data member. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db index
  std::string name_;
};
  </pre>

  <p>For more information on defining database indexes, refer to
     <a href="#14.7">Section 14.7, "Index Definition Pragmas"</a>.</p>

  <h3><a name="14.4.18">14.4.18 <code>unique</code></a></h3>

  <p>The <code>index</code> specifier instructs the ODB compiler to define
     a unique database index for the data member. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db unique
  std::string name_;
};
  </pre>

  <p>For more information on defining database indexes, refer to
     <a href="#14.7">Section 14.7, "Index Definition Pragmas"</a>.</p>

  <h3><a name="14.4.19">14.4.19 <code>unordered</code></a></h3>

  <p>The <code>unordered</code> specifier specifies that the member of
     an ordered container type should be stored unordered in the database.
     The database table for such a member will not contain the index column
     and the order in which elements are retrieved from the database may
     not be the same as the order in which they were stored. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db unordered
  std::vector&lt;std::string> nicknames_;
};
  </pre>

  <p>For a more detailed discussion of ordered containers and their
     storage in the database, refer to <a href="#5.1">Section 5.1,
     "Ordered Containers"</a>.</p>

  <h3><a name="14.4.20">14.4.20 <code>table</code></a></h3>

  <p>The <code>table</code> specifier specifies the table name that should
     be used to store the contents of the container member. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db table("nicknames")
  std::vector&lt;std::string> nicknames_;
};
  </pre>

  <p>If the table name is not specified, then the container table name
     is constructed by concatenating the object's table name, underscore,
     and the public member name. The public member name is obtained
     by removing the common member name decorations, such as leading and
     trailing underscores, the <code>m_</code> prefix, etc. In the example
     above, without the <code>table</code> specifier, the container's
     table name would have been <code>person_nicknames</code>.</p>

  <p>The <code>table</code> specifier can also be used for members of
     composite value types. In this case it specifies the table name
     prefix for container members inside the composite value type. Refer
     to <a href="#7.2.2">Section 7.2.2, "Composite Value Column and Table
     Names"</a> for details.</p>

  <p>The container table name can be qualified with a database
     schema, for example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db table("extras.nicknames")
  std::vector&lt;std::string> nicknames_;
};
  </pre>

  <p>For more information on database schemas and the format of the
     qualified names, refer to <a href="#14.1.8">Section 14.1.8,
     "<code>schema</code>"</a>.</p>

  <h3><a name="14.4.21">14.4.21 <code>load</code>/<code>update</code></a></h3>

  <p>The <code>load</code> and <code>update</code> specifiers specify the
     loading and updating behavior for an object section, respectively.
     Valid values for the <code>load</code> specifier are
     <code>eager</code> (default) and <code>lazy</code>. Valid values for
     the <code>update</code> specifier are <code>always</code> (default),
     <code>change</code>, and <code>manual</code>. For more information
     on object sections, refer to <a href="#9">Chapter 9, "Sections"</a>.</p>

  <h3><a name="14.4.22">14.4.22 <code>section</code></a></h3>

  <p>The <code>section</code> specifier indicates that a data member
     of a persistent class belongs to an object section. The single
     required argument to this specifier is the name of the section
     data member. This specifier can only be used on direct data
     members of a persistent class. For more information on object
     sections, refer to <a href="#9">Chapter 9, "Sections"</a>.</p>

  <h3><a name="14.4.23">14.4.23 <code>added</code></a></h3>

  <p>The <code>added</code> specifier marks the data member as
     soft-added. The single required argument to this specifier is
     the addition version. For more information on this functionality,
     refer to <a href="#13.4">Section 13.4, "Soft Object Model
     Changes"</a>.</p>

  <h3><a name="14.4.24">14.4.24 <code>deleted</code></a></h3>

  <p>The <code>deleted</code> specifier marks the data member as
     soft-deleted. The single required argument to this specifier is
     the deletion version. For more information on this functionality,
     refer to <a href="#13.4">Section 13.4, "Soft Object Model
     Changes"</a>.</p>

  <h3><a name="14.4.25">14.4.25 <code>index_type</code></a></h3>

  <p>The <code>index_type</code> specifier specifies the native
     database type that should be used for an ordered container's
     index column of the data member. The semantics of <code>index_type</code>
     are similar to those of the <code>type</code> specifier
     (<a href="#14.4.3">Section 14.4.3, "<code>type</code>"</a>). The native
     database type is expected to be an integer type. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db index_type("SMALLINT UNSIGNED")
  std::vector&lt;std::string> nicknames_;
};
  </pre>

  <h3><a name="14.4.26">14.4.26 <code>key_type</code></a></h3>

  <p>The <code>key_type</code> specifier specifies the native
     database type that should be used for a map container's
     key column of the data member. The semantics of <code>key_type</code>
     are similar to those of the <code>type</code> specifier
     (<a href="#14.4.3">Section 14.4.3, "<code>type</code>"</a>). For
     example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db key_type("INT UNSIGNED")
  std::map&lt;unsigned short, float> age_weight_map_;
};
  </pre>

  <h3><a name="14.4.27">14.4.27 <code>value_type</code></a></h3>

  <p>The <code>value_type</code> specifier specifies the native
     database type that should be used for a container's
     value column of the data member. The semantics of <code>value_type</code>
     are similar to those of the <code>type</code> specifier
     (<a href="#14.4.3">Section 14.4.3, "<code>type</code>"</a>). For
     example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db value_type("VARCHAR(255)")
  std::vector&lt;std::string> nicknames_;
};
  </pre>

  <p>The <code>value_null</code> and <code>value_not_null</code>
     (<a href="#14.4.28">Section 14.4.28,
     "<code>value_null</code>/<code>value_not_null</code>"</a>) specifiers
     can be used to control the <code>NULL</code> semantics of a value
     column.</p>

  <h3><a name="14.4.28">14.4.28 <code>value_null</code>/<code>value_not_null</code></a></h3>

  <p>The <code>value_null</code> and <code>value_not_null</code> specifiers
     specify that a container's element value for the data member can or
     cannot be <code>NULL</code>, respectively. The semantics of
     <code>value_null</code> and <code>value_not_null</code> are similar
     to those of the <code>null</code> and <code>not_null</code> specifiers
     (<a href="#14.4.6">Section 14.4.6, "<code>null</code>/<code>not_null</code>"</a>).
     For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
};

#pragma db object
class account
{
  ...

  #pragma db value_not_null
  std::vector&lt;std::shared_ptr&lt;person>> holders_;
};
  </pre>

  <p>For set and multiset containers (<a href="#5.2">Section 5.2, "Set and
     Multiset Containers"</a>) the element value is automatically treated
     as not allowing a <code>NULL</code> value.</p>

  <h3><a name="14.4.29">14.4.29 <code>id_options</code></a></h3>

  <p>The <code>id_options</code> specifier specifies additional
     column definition options that should be used for a container's
     id column of the data member. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db id options("COLLATE binary")
  std::string name_;

  #pragma db id_options("COLLATE binary")
  std::vector&lt;std::string> nicknames_;
};
  </pre>

  <p>The semantics of <code>id_options</code> are similar to those
     of the <code>options</code> specifier (<a href="#14.4.8">Section
     14.4.8, "<code>options</code>"</a>).</p>

  <h3><a name="14.4.30">14.4.30 <code>index_options</code></a></h3>

  <p>The <code>index_options</code> specifier specifies additional
     column definition options that should be used for a container's
     index column of the data member. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db index_options("ZEROFILL")
  std::vector&lt;std::string> nicknames_;
};
  </pre>

  <p>The semantics of <code>index_options</code> are similar to those
     of the <code>options</code> specifier (<a href="#14.4.8">Section
     14.4.8, "<code>options</code>"</a>).</p>

  <h3><a name="14.4.31">14.4.31 <code>key_options</code></a></h3>

  <p>The <code>key_options</code> specifier specifies additional
     column definition options that should be used for a container's
     key column of the data member. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db key_options("COLLATE binary")
  std::map&lt;std::string, std::string> properties_;
};
  </pre>

  <p>The semantics of <code>key_options</code> are similar to those
     of the <code>options</code> specifier (<a href="#14.4.8">Section
     14.4.8, "<code>options</code>"</a>).</p>

  <h3><a name="14.4.32">14.4.32 <code>value_options</code></a></h3>

  <p>The <code>value_options</code> specifier specifies additional
     column definition options that should be used for a container's
     value column of the data member. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db value_options("COLLATE binary")
  std::set&lt;std::string> nicknames_;
};
  </pre>

  <p>The semantics of <code>value_options</code> are similar to those
     of the <code>options</code> specifier (<a href="#14.4.8">Section
     14.4.8, "<code>options</code>"</a>).</p>

  <h3><a name="14.4.33">14.4.33 <code>id_column</code></a></h3>

  <p>The <code>id_column</code> specifier specifies the column
     name that should be used to store the object id in a
     container's table for the data member. The semantics of
     <code>id_column</code> are similar to those of the
     <code>column</code> specifier
     (<a href="#14.4.9">Section 14.4.9, "<code>column</code>"</a>).
     For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db id_column("person_id")
  std::vector&lt;std::string> nicknames_;
};
  </pre>

  <p>If the column name is not specified, then <code>object_id</code>
     is used by default.</p>

  <h3><a name="14.4.34">14.4.34 <code>index_column</code></a></h3>

  <p>The <code>index_column</code> specifier specifies the column
     name that should be used to store the element index in an
     ordered container's table for the data member. The semantics of
     <code>index_column</code> are similar to those of the
     <code>column</code> specifier
     (<a href="#14.4.9">Section 14.4.9, "<code>column</code>"</a>).
     For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db index_column("nickname_number")
  std::vector&lt;std::string> nicknames_;
};
  </pre>

  <p>If the column name is not specified, then <code>index</code>
     is used by default.</p>

  <h3><a name="14.4.35">14.4.35 <code>key_column</code></a></h3>

  <p>The <code>key_column</code> specifier specifies the column
     name that should be used to store the key in a map
     container's table for the data member. The semantics of
     <code>key_column</code> are similar to those of the
     <code>column</code> specifier
     (<a href="#14.4.9">Section 14.4.9, "<code>column</code>"</a>).
     For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db key_column("age")
  std::map&lt;unsigned short, float> age_weight_map_;
};
  </pre>

  <p>If the column name is not specified, then <code>key</code>
     is used by default.</p>

  <h3><a name="14.4.36">14.4.36 <code>value_column</code></a></h3>

  <p>The <code>value_column</code> specifier specifies the column
     name that should be used to store the element value in a
     container's table for the data member. The semantics of
     <code>value_column</code> are similar to those of the
     <code>column</code> specifier
     (<a href="#14.4.9">Section 14.4.9, "<code>column</code>"</a>).
     For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db value_column("weight")
  std::map&lt;unsigned short, float> age_weight_map_;
};
  </pre>

  <p>If the column name is not specified, then <code>value</code>
     is used by default.</p>

  <h3><a name="14.4.37">14.4.37 <code>points_to</code></a></h3>

  <p>The <code>points_to</code> specifier allows the establishment of
     object relationships without using object pointers. For example:</p>

<pre class="cxx">

#pragma db object
class employer
{
  ...

  #pragma db id
  std::string id_;
};

#pragma db object
class person
{
  ...

  #pragma db points_to(employer)
  std::string employer_;
};
  </pre>

  <h2><a name="14.5">14.5 Namespace Pragmas</a></h2>

  <p>A pragma with the <code>namespace</code> qualifier describes a
     C++ namespace. Similar to other qualifiers, <code>namespace</code>
     can also refer to a named C++ namespace, for example:</p>

  <pre class="cxx">
namespace test
{
  ...
}

#pragma db namespace(test) ...
  </pre>

  <p>To refer to the global namespace in the <code>namespace</code>
     qualifier the following special syntax is used:</p>

  <pre class="cxx">
#pragma db namespace() ....
  </pre>

  <p>The <code>namespace</code> qualifier can be optionally followed,
     in any order, by one or more specifiers summarized in the
     table below:</p>

  <!-- border="1" is necessary for html2ps -->
  <table class="specifiers" border="1">
    <tr>
      <th>Specifier</th>
      <th>Summary</th>
      <th>Section</th>
    </tr>

    <tr>
      <td><code>pointer</code></td>
      <td>pointer type for persistent classes and views inside a namespace</td>
      <td><a href="#14.5.1">14.5.1</a></td>
    </tr>

    <tr>
      <td><code>table</code></td>
      <td>table name prefix for persistent classes inside a namespace</td>
      <td><a href="#14.5.2">14.5.2</a></td>
    </tr>

    <tr>
      <td><code>schema</code></td>
      <td>database schema for persistent classes inside a namespace</td>
      <td><a href="#14.5.3">14.5.3</a></td>
    </tr>

    <tr>
      <td><code>session</code></td>
      <td>enable/disable session support for persistent classes inside a namespace</td>
      <td><a href="#14.5.4">14.5.4</a></td>
    </tr>

  </table>

  <h3><a name="14.5.1">14.5.1 <code>pointer</code></a></h3>

  <p>The <code>pointer</code> specifier specifies the default pointer
     type for persistent classes and views inside the namespace. For
     example:</p>

  <pre class="cxx">
#pragma db namespace pointer(std::shared_ptr)
namespace accounting
{
  #pragma db object
  class employee
  {
    ...
  };

  #pragma db object
  class employer
  {
    ...
  };
}
  </pre>

  <p>There are only two valid ways to specify a pointer with the
     <code>pointer</code> specifier at the namespace level. We can
     specify the template name of a smart pointer in which
     case the ODB compiler will automatically append the class
     name as a template argument. Or we can use <code>*</code>
     to denote a raw pointer.</p>

  <p>Note also that we can always override the default pointer
     specified at the namespace level for any persistent class
     or view inside this namespace. For example:</p>

  <pre class="cxx">
#pragma db namespace pointer(std::unique_ptr)
namespace accounting
{
  #pragma db object pointer(std::shared_ptr)
  class employee
  {
    ...
  };

  #pragma db object
  class employer
  {
    ...
  };
}
  </pre>

  <p>For a more detailed discussion of object and view pointers, refer
     to <a href="#3.3">Section 3.3, "Object and View Pointers"</a>.</p>

  <h3><a name="14.5.2">14.5.2 <code>table</code></a></h3>

  <p>The <code>table</code> specifier specifies a table prefix
     that should be added to table names of persistent classes inside
     the namespace. For example:</p>

  <pre class="cxx">
#pragma db namespace table("acc_")
namespace accounting
{
  #pragma db object table("employees")
  class employee
  {
    ...
  };

  #pragma db object table("employers")
  class employer
  {
    ...
  };
}
  </pre>

  <p>In the above example the resulting table names will be
     <code>acc_employees</code> and <code>acc_employers</code>.</p>

  <p>The table name prefix can also be specified with the
     <code>--table-prefix</code> ODB compiler option. Note
     that table prefixes specified at the namespace level as well
     as with the command line option are accumulated. For example:</p>

  <pre class="cxx">
#pragma db namespace() table("audit_")

#pragma db namespace table("hr_")
namespace hr
{
  #pragma db object table("employees")
  class employee
  {
    ...
  };
}

#pragma db object table("employers")
class employer
{
  ...
};
  </pre>

  <p>If we compile the above example with the
     <code>--table-prefix&nbsp;test_</code> option, then the
     <code>employee</code> class table will be called
     <code>test_audit_hr_employees</code> and <code>employer</code> &mdash;
     <code>test_audit_employers</code>.</p>

  <p>Table prefixes can be used as an alternative to database schemas
     (<a href="#14.1.8">Section 14.1.8, "<code>schema</code>"</a>) if
     the target database system does not support schemas.</p>

  <h3><a name="14.5.3">14.5.3 <code>schema</code></a></h3>

  <p>The <code>schema</code> specifier specifies a database schema
     that should be used for persistent classes inside the namespace.
     For more information on specifying a database schema refer to
     <a href="#14.1.8">Section 14.1.8, "<code>schema</code>"</a>.</p>

  <h3><a name="14.5.4">14.5.4 <code>session</code></a></h3>

  <p>The <code>session</code> specifier specifies whether to enable
     session support for persistent classes inside the namespace. For
     example:</p>

  <pre class="cxx">
#pragma db namespace session
namespace hr
{
  #pragma db object                // Enabled.
  class employee
  {
    ...
  };

  #pragma db object session(false) // Disabled.
  class employer
  {
    ...
  };
}
  </pre>

  <p>Session support is disabled by default unless the
     <code>--generate-session</code> ODB compiler option is specified.
     Session support specified at the namespace level can be overridden
     on the per object basis (<a href="#14.1.10">Section 14.1.10,
     "<code>session</code>"</a>). For more information on sessions,
     refer to <a href="#11">Chapter 11, "Session"</a>.</p>

<h2><a name="14.6">14.6 Object Model Pragmas</a></h2>

  <p>A pragma with the <code>model</code> qualifier describes the
     whole C++ object model. For example:</p>

  <pre class="cxx">
#pragma db model ...
  </pre>

  <p>The <code>model</code> qualifier can be followed, in any order,
     by one or more specifiers summarized in the table below:</p>

  <!-- border="1" is necessary for html2ps -->
  <table class="specifiers" border="1">
    <tr>
      <th>Specifier</th>
      <th>Summary</th>
      <th>Section</th>
    </tr>

    <tr>
      <td><code>version</code></td>
      <td>object model version</td>
      <td><a href="#14.6.1">14.6.1</a></td>
    </tr>

  </table>

  <h3><a name="14.6.1">14.6.1 <code>version</code></a></h3>

  <p>The <code>version</code> specifier specifies the object model
     version when schema evolution support is used. The first two
     required arguments to this specifier are the base and current
     model versions, respectively. The third optional argument
     specifies whether the current version is open for changes.
     Valid values for this argument are <code>open</code> (the
     default) and <code>closed</code>. For more information on
     this functionality, refer to <a href="#13">Chapter 13,
     "Database Schema Evolution"</a>.</p>


  <h2><a name="14.7">14.7 Index Definition Pragmas</a></h2>

  <p>While it is possible to manually add indexes to the generated
     database schema, it is more convenient to do this as part of
     the persistent class definitions. A pragma with the <code>index</code>
     qualifier describes a database index. It has the following
     general format:</p>

<pre class="cxx">
#pragma db index[("&lt;name>")]                       \
           [unique|type("&lt;type>")]                 \
           [method("&lt;method>")]                    \
           [options("&lt;index-options>")]            \
           member(&lt;name>[, "&lt;column-options>"])... \
           members(&lt;name>[,&lt;name>...])...
</pre>

  <p>The <code>index</code> qualifier can optionally specify the
     index name. If the index name is not specified, then one is
     automatically derived by appending the <code>_i</code> suffix
     to the column name of the index member. If the name is not
     specified and the index contains multiple members, then the
     index definition is invalid.</p>

  <p>The optional <code>type</code>, <code>method</code>, and
     <code>options</code> clauses specify the index type, for
     example <code>UNIQUE</code>, index method, for example
     <code>BTREE</code>, and index options, respectively. The
     <code>unique</code> clause is a shortcut for
     <code>type("UNIQUE")</code>. Note that not all database
     systems support specifying an index method or options.
     For more information on the database system-specific index
     types, methods, and options, refer to <a href="#II">Part II,
     "Database Systems"</a>.</p>

  <p>To specify index members we can use the <code>member</code>
     or <code>members</code> clauses, or a mix of the two. The
     <code>member</code> clause allows us to specify a single
     index member with optional column options, for example,
     <code>"ASC"</code>. If we need to create a composite
     index that contains multiple members, then we can repeat
     the <code>member</code> clause several times or, if the
     members don't have any column options, we can use a single
     <code>members</code> clause instead. Similar to the index
     type, method, and options, the format of column options is
     database system-specific. For more details, refer to
     <a href="#II">Part II, "Database Systems"</a>.</p>

  <p>The following code fragment shows some typical examples
     of index definitions:</p>

<pre class="cxx">
#pragma db object
class object
{
  ...

  int x;
  int y;
  int z1;
  int z2;

  // An index for member x with automatically-assigned name x_i.
  //
  #pragma db index member(x)

  // A unique index named y_index for member y which is sorted in
  // the descending order. The index is using the BTREE method.
  //
  #pragma db index("y_index") unique method("BTREE") member(y, "DESC")

  // A composite BITMAP index named z_i for members z1 and z2.
  //
  #pragma db index("z_i") type("BITMAP") members(z1, z2)
};
</pre>

  <p>ODB also offers a shortcut for defining an index with the default
     method and options for a single data member. Such an index can
     be defined using the <code>index</code> (<a href="#14.4.17">Section
     14.4.17, "<code>index</code>"</a>) or <code>unique</code>
     (<a href="#14.4.18">Section 14.4.18, "<code>unique</code>"</a>)
     member specifier. For example:</p>

<pre class="cxx">
#pragma db object
class object
{
  ...

  #pragma db index
  int x;

  #pragma db type("INT") unique
  int y;
};
</pre>

  <p>The above example is semantically equivalent to the following
     more verbose version:</p>

<pre class="cxx">
#pragma db object
class object
{
  ...

  int x;

  #pragma db type("INT")
  int y;

  #pragma db index member(x)
  #pragma db index unique member(y)
};
</pre>

  <p>While it is convenient to define an index inside a persistent
     class, it is also possible to do that out of the class body. In this
     case, the index name, if specified, must be prefixed with the
     potentially-qualified class name. For example:</p>

<pre class="cxx">
namespace n
{
  #pragma db object
  class object
  {
    ...

    int x;
    int y;
  };

  // An index for member x in persistent class object with automatically-
  // assigned name x_i.
  //
  #pragma db index(object) member(x)
}

// An index named y_index for member y in persistent class n::object.
//
#pragma db index(n::object::"y_index") member(y)
</pre>

  <p>It is possible to define an index on a member that is of a
     composite value type or on some of its nested members. For
     example:</p>

<pre class="cxx">
#pragma db value
struct point
{
  int x;
  int y;
  int z;
};

#pragma db object
class object
{
  // An index that includes all of the p1's nested members.
  //
  #pragma db index
  point p1;

  point p2;

  // An index that includes only p2.x and p2.y.
  //
  #pragma db index("p2_xy_i") members(p2.x, p2.y)
};
</pre>

  <p>When generating a schema for a container member (<a href="#5">Chapter 5,
     "Containers"</a>), ODB automatically defines two indexes on the container
     table. One is for the object id that references the object table and the
     other is for the index column in case the container is ordered
     (<a href="#5.1">Section 5.1, "Ordered Containers"</a>). By default these
     indexes use the default index name, type, method, and options. The
     <code>index</code> pragma allows us to customize these two indexes by
     recognizing the special <code>id</code> and <code>index</code> nested
     member names when specified after a container member. For example:</p>

<pre class="cxx">
#pragma db object
class object
{
  std::vector&lt;int> v;

  // Change the container id index name.
  //
  #pragma db index("id_index") member(v.id)

  // Change the container index index method.
  //
  #pragma db index method("BTREE") member(v.index)
};
</pre>

  <h2><a name="14.8">14.8 Database Type Mapping Pragmas</a></h2>

  <p>A pragma with the <code>map</code> qualifier describes a mapping between
     either two C++ types (<a href="#14.8.1">Section 14.8.1, "C++ Type Mapping
     Pragmas"</a>) or two database types (<a href="#14.8.2">Section 14.8.2,
     "Database Type Mapping Pragmas"</a>).</p>

  <h3><a name="14.8.1">14.8.1 C++ Type Mapping Pragmas</a></h3>

  <p>A pragma with the <code>map</code> qualifier can describe a mapping
     between two C++ types. For each database system ODB provides built-in
     support for mapping a standard set of C++ types, such as integers,
     strings, binary, etc. However, many codebases may use custom versions of
     corresponding (or similar) C++ types. While it is possible to add support
     for such custom types by providing a suitable specialization of the
     <code>value_traits</code> class template, it is often simpler to define
     their mapping in terms of one of the already supported types by
     specifying the conversion between the two types. This mechanism can also
     be used to redefine the mapping for one of the standard types. For
     example, we could map <code>bool</code> to <code>std::string</code> in
     order to save boolean values as string literals. This mechanism can also
     be used with composite value types.</p>

  <p>The <code>map</code> pragma for C++ types has the following format:</p>

<pre class="cxx">
#pragma db map type(&lt;name>)  \
               as(&lt;name>)    \
               [to(&lt;expr>)]  \
               [from(&lt;expr>)]
</pre>

  <p>The <code>type</code> clause specifies the name of the C++ type
     that we are mapping. We will refer to it as the <em>mapped type</em>
     from now on.</p>

  <p>The <code>as</code> clause specifies the name of the C++ type that we are
     mapping the mapped type to. We will refer to it as the <em>interface
     type</em> from now on.</p>

  <p>The optional <code>to</code> and <code>from</code> clauses specify the
     C++ conversion expressions between the mapped type and the
     interface type. The <code>to</code> expression converts from the
     interface type to the mapped type and <code>from</code> converts
     in the other direction. If no explicit conversion is required for
     either direction, then the corresponding clause can be omitted.

     If the conversion expressions are specified, then they must contain the
     special <code>(?)</code> placeholder which will be replaced with the
     actual value to be converted.</p>

  <p>As an example, suppose we have the <code>path</code> type which
     represents a filesystem path and which we wish to store in the
     database. Suppose also that it has a constructor that allows implicit
     conversion of <code>std::string</code> to <code>path</code> as well as
     the <code>string()</code> member function which returns the
     <code>std::string</code> representation of <code>path</code>. This is
     how we can map <code>path</code> to <code>std::string</code> to be able
     to store it in the database:</p>

<pre class="cxx">
#pragma db map type(path)        \
               as(std::string)   \
               from((?).string ())
</pre>

  <p>Notice that we could omit the <code>to</code> expression because
     <code>std::string</code> can be implicitly converted to
     <code>path</code>. In this case the implied expression is equivalent
     to <code>to((?))</code>.</p>

  <p>As a more elaborate example, the following <code>map</code> pragma maps
     the standard <code>chrono</code> <code>time_point</code> type (that is, a
     timestamp) as <code>uint64</code> with the stored values representing the
     number of nanoseconds since the UNIX epoch:</p>

<pre class="cxx">
#pragma db map                                                        \
  type(std::chrono::system_clock::time_point)                         \
  as(std::uint64_t)                                                   \
  to(std::chrono::duration_cast&lt;std::chrono::nanoseconds> (           \
       (?).time_since_epoch ()).count ())                             \
  from(std::chrono::system_clock::time_point (                        \
    std::chrono::duration_cast&lt;std::chrono::system_clock::duration> ( \
      std::chrono::nanoseconds (?))))
</pre>

  <p>And the following example shows how to redefine the mapping for one of
     the standard types:</p>

<pre class="cxx">
#pragma db map type(bool)                 \
               as(std::string)            \
               to((?) ? "true" : "false") \
               from((?) == "true")
</pre>

  <h3><a name="14.8.2">14.8.2 Database Type Mapping Pragmas</a></h3>

  <p>A pragma with the <code>map</code> qualifier can describe a
     mapping between two database types. For each database system
     ODB provides built-in support for a core set of database types,
     such as integers, strings, binary, etc. However, many database
     systems provide extended types such as geospatial types,
     user-defined types, and collections (arrays, table types,
     key-value stores, etc). In order to support such extended types,
     ODB allows us to map them to one of the built-in types, normally
     a string or a binary. Given the text or binary representation
     of the data we can then extract it into our chosen C++ data type
     and thus establish a mapping between an extended database type and
     its C++ equivalent.</p>

  <p>The <code>map</code> pragma for database types has the following
  format:</p>

<pre class="cxx">
#pragma db map type("&lt;regex>")  \
               as("&lt;subst>")    \
               [to("&lt;subst>")]  \
               [from("&lt;subst>")]
</pre>

  <p>The <code>type</code> clause specifies the name of the database type
     that we are mapping. We will refer to it as the <em>mapped type</em>
     from now on. The name of the mapped type is a Perl-like regular
     expression pattern that is matched in the case-insensitive mode.</p>

  <p>The <code>as</code> clause specifies the name of the database type
     that we are mapping the mapped type to. We will refer to it as
     the <em>interface type</em> from now on. The name of the interface
     type is a regular expression substitution and should expand to a
     name of a database type for which ODB provides built-in support.</p>

  <p>The optional <code>to</code> and <code>from</code> clauses specify the
     database conversion expressions between the mapped type and the
     interface type. The <code>to</code> expression converts from the
     interface type to the mapped type and <code>from</code> converts
     in the other direction. If no explicit conversion is required for
     either direction, then the corresponding clause can be omitted.
     The conversion expressions are regular expression substitutions.
     They must contain the special <code>(?)</code> placeholder which will
     be replaced with the actual value to be converted. Turning on SQL
     statement tracing (<a href="#3.13">Section 3.13, "Tracing SQL
     Statement Execution"</a>) can be useful for debugging conversion
     expressions. This allows you to see the substituted expressions
     as used in the actual statements.</p>

  <p>As an example, the following <code>map</code> pragma maps the
     PostgreSQL array of <code>INTEGER</code>'s to <code>TEXT</code>:</p>

<pre class="cxx">
#pragma db map type("INTEGER *\\[(\\d*)\\]") \
               as("TEXT")                    \
               to("(?)::INTEGER[$1]")        \
               from("(?)::TEXT")
</pre>

  <p>With the above mapping we can now have a persistent class that
     has a member of the PostgreSQL array type:</p>

<pre class="cxx">
#pragma db object
class object
{
  ...

  #pragma db type("INTEGER[]")
  std::string array_;
};
</pre>

  <p>In PostgreSQL the array literal has the <code>{n1,n2,...}</code> form.
     As a result, we need to make sure that we pass the correct text
     representation in the <code>array_</code> member, for example:</p>

<pre class="cxx">
object o;
o.array_ = "{1,2,3}";
db.persist (o);
</pre>

  <p>Of course, <code>std::string</code> is not the most natural
     representation of an array of integers in C++. Instead,
     <code>std::vector&lt;int></code> would have been much more
     appropriate. To add support for mapping
     <code>std::vector&lt;int></code> to PostgreSQL <code>INTEGER[]</code>
     we need to provide a <code>value_traits</code> specialization
     that implements conversion between the PostgreSQL text representation
     of an array and <code>std::vector&lt;int></code>. Below is a sample
     implementation:</p>

<pre class="cxx">
namespace odb
{
  namespace pgsql
  {
    template &lt;>
    class value_traits&lt;std::vector&lt;int>, id_string>
    {
    public:
      using value_type = std::vector&lt;int>;
      using query_type = value_type;
      using image_type = details::buffer;

      static void
      set_value (value_type&amp; v,
                 const details::buffer&amp; b,
                 std::size_t n,
                 bool is_null)
      {
        v.clear ();

        if (!is_null)
        {
          char c;
          std::istringstream is (std::string (b.data (), n));

          is >> c; // '{'

          for (c = static_cast&lt;char> (is.peek ()); c != '}'; is >> c)
          {
            v.push_back (int ());
            is >> v.back ();
          }
        }
      }

      static void
      set_image (details::buffer&amp; b,
                 std::size_t&amp; n,
                 bool&amp; is_null,
                 const value_type&amp; v)
      {
        is_null = false;
        std::ostringstream os;

        os &lt;&lt; '{';

        for (value_type::const_iterator i (v.begin ()), e (v.end ());
             i != e;)
        {
          os &lt;&lt; *i;

          if (++i != e)
            os &lt;&lt; ',';
        }

        os &lt;&lt; '}';

        const std::string&amp; s (os.str ());
        n = s.size ();

        if (n > b.capacity ())
          b.capacity (n);

        std::memcpy (b.data (), s.c_str (), n);
      }
    };
  }
}
</pre>

  <p>Once this specialization is included in the generated code (see
     the <code>mapping</code> example in the <code>odb-examples</code>
     package for details), we can use <code>std::vector&lt;int></code>
     instead of <code>std::string</code> in our persistent class:</p>

<pre class="cxx">
#pragma db object
class object
{
  ...

  #pragma db type("INTEGER[]")
  std::vector&lt;int> array_;
};
</pre>

  <p>If we wanted to always map <code>std::vector&lt;int></code>
     to PostgreSQL <code>INTEGER[]</code>, then we could instead
     write:</p>

<pre class="cxx">
using int_vector = std::vector&lt;int>;
#pragma db value(int_vector) type("INTEGER[]")

#pragma db object
class object
{
  ...

  std::vector&lt;int> array_; // Mapped to INTEGER[].
};
</pre>

  <p>While the above example only shows how to handle PostgreSQL arrays,
     other types in PostgreSQL and in other databases can be supported
     in a similar way. The <code>odb-tests</code> package contains a
     set of tests in the <code>&lt;database>/custom</code> directories that,
     for each database, shows how to provide custom mapping for some of
     the extended types.</p>

  <h2><a name="14.9">14.9 C++ Compiler Warnings</a></h2>

  <p>When a C++ header file defining persistent classes and containing
     ODB pragmas is used to build the application, the C++ compiler may
     issue warnings about pragmas that it doesn't recognize. There
     are several ways to deal with this problem. The easiest is to
     disable such warnings using one of the compiler-specific command
     line options or warning control pragmas. This method is described
     in the following sub-section for popular C++ compilers.</p>

  <p>There are also several C++ compiler-independent methods that we
     can employ. The first is to use the <code>PRAGMA_DB</code> macro,
     defined in <code>&lt;odb/core.hxx></code>, instead of using
     <code>#pragma&nbsp;db</code> directly. This macro expands to the
     ODB pragma when compiled with the ODB compiler and to an empty
     declaration when compiled with other compilers. The following example
     shows how we can use this macro:</p>

  <pre class="cxx">
#include &lt;odb/core.hxx>

PRAGMA_DB(object)
class person
{
  ...

  PRAGMA_DB(id)
  unsigned long long id_;
};
  </pre>

  <p>An alternative to using the <code>PRAGMA_DB</code> macro is to
     group the <code>#pragma&nbsp;db</code> directives in blocks that are
     conditionally included into compilation only when compiled with the
     ODB compiler. For example:</p>

  <pre class="cxx">
class person
{
  ...

  unsigned long long id_;
};

#ifdef ODB_COMPILER
#  pragma db object(person)
#  pragma db member(person::id_) id
#endif
  </pre>

  <p>The disadvantage of this approach is that it can quickly become
     overly verbose when positioned pragmas are used.</p>

  <h3><a name="14.9.1">14.9.1 GNU C++</a></h3>

  <p>GNU g++ does not issue warnings about unknown pragmas
     unless requested with the <code>-Wall</code> command line option.
     To disable only the unknown pragma warning, we can add the
     <code>-Wno-unknown-pragmas</code> option after <code>-Wall</code>,
     for example:</p>

  <pre class="terminal">
g++ -Wall -Wno-unknown-pragmas ...
  </pre>

  <h3><a name="14.9.2">14.9.2 Visual C++</a></h3>

  <p>Microsoft Visual C++ issues an unknown pragma warning (C4068) at
     warning level 1 or higher. This means that unless we have disabled
     the warnings altogether (level 0), we will see this warning.</p>

  <p>To disable this warning via the compiler command line, we can add
     the <code>/wd4068</code> C++ compiler option in Visual Studio 2008
     and earlier. In Visual Studio 2010 and later there is now a special
     GUI field where we can enter warning numbers that should be disabled.
     Simply enter 4068 into this field.</p>

  <p>We can also disable this warning for only a specific header or
     a fragment of a header using the warning control pragma. For
     example:</p>

  <pre class="cxx">
#include &lt;odb/core.hxx>

#pragma warning (push)
#pragma warning (disable:4068)

#pragma db object
class person
{
  ...

  #pragma db id
  unsigned long long id_;
};

#pragma warning (pop)
  </pre>

  <h3><a name="14.9.3">14.9.3 Sun C++</a></h3>

  <p>The Sun C++ compiler does not issue warnings about unknown pragmas
     unless the <code>+w</code> or <code>+w2</code> option is specified.
     To disable only the unknown pragma warning we can add the
     <code>-erroff=unknownpragma</code> option anywhere on the
     command line, for example:</p>

  <pre class="terminal">
CC +w -erroff=unknownpragma ...
  </pre>

  <h3><a name="14.9.4">14.9.4 IBM XL C++</a></h3>

  <p>IBM XL C++ issues an unknown pragma warning (1540-1401) by default.
     To disable this warning we can add the <code>-qsuppress=1540-1401</code>
     command line option, for example:</p>

  <pre class="terminal">
xlC -qsuppress=1540-1401 ...
  </pre>

  <h3><a name="14.9.5">14.9.5 HP aC++</a></h3>

  <p>HP aC++ (aCC) issues an unknown pragma warning (2161) by default.
     To disable this warning we can add the <code>+W2161</code>
     command line option, for example:</p>

  <pre class="terminal">
aCC +W2161 ...
  </pre>

  <h3><a name="14.9.6">14.9.6 Clang</a></h3>

  <p>Clang does not issue warnings about unknown pragmas
     unless requested with the <code>-Wall</code> command line option.
     To disable only the unknown pragma warning, we can add the
     <code>-Wno-unknown-pragmas</code> option after <code>-Wall</code>,
     for example:</p>

  <pre class="terminal">
clang++ -Wall -Wno-unknown-pragmas ...
  </pre>

  <p>We can also disable this warning for only a specific header or
     a fragment of a header using the warning control pragma. For
     example:</p>

  <pre class="cxx">
#include &lt;odb/core.hxx>

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunknown-pragmas"

#pragma db object
class person
{
  ...

  #pragma db id
  unsigned long long id_;
};

#pragma clang diagnostic pop
  </pre>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="15">15 Advanced Techniques and Mechanisms</a></h1>

  <p>This chapter covers more advanced techniques and mechanisms
     provided by ODB that may be useful in certain situations.</p>

  <h2><a name="15.1">15.1 Transaction Callbacks</a></h2>

  <p>The ODB transaction class (<code>odb::transaction</code>) allows
     an application to register a callback that will be called after
     the transaction is finalized, that is, committed or rolled back.
     This mechanism can be used, for example, to restore values that
     were updated during the transaction execution to their original
     states if the transaction is rolled back.</p>

  <p>The callback management interface of the <code>transaction</code>
     class is shown below.</p>

  <pre class="cxx">
namespace odb
{
  class transaction
  {
    ...

  public:
    static const unsigned short event_commit = 0x01;
    static const unsigned short event_rollback = 0x02;
    static const unsigned short event_all = event_commit | event_rollback;

    using callback_type = void (*) (
      unsigned short event, void* key, unsigned long long data);

    void
    callback_register (callback_type callback,
                       void* key,
                       unsigned short event = event_all,
                       unsigned long long data = 0,
                       transaction** state = 0);


    void
    callback_unregister (void* key);

    void
    callback_update (void* key,
                     unsigned short event,
                     unsigned long long data = 0,
                     transaction** state = 0);
  }
}
  </pre>

  <p>The <code>callback_register()</code> function registers a
     post-commit/rollback callback. The <code>callback</code>
     argument is the function that should be called. The
     <code>key</code> argument is used by the transaction
     to identify this callback. It is also normally used
     to pass an address of the data object on which the
     callback function will work. The <code>event</code>
     argument is the bitwise-or of the events that should
     trigger the callback.</p>

  <p>The optional data argument can be used to store any POD
     user data that doesn't exceed 8 bytes in size and doesn't require
     alignment greater than <code>unsigned long long</code>. For
     example, we could store an old value of a flag or a counter
     that needs to be restored in case of a roll back.</p>

  <p>The optional <code>state</code> argument can be used to
     indicate that the callback has been unregistered because
     the transaction was finalized. In this case the transaction
     automatically resets the passed pointer to 0. This is
     primarily useful if we are interested in only one of
     the events (commit or rollback).</p>

  <p>The <code>callback_unregister()</code> function unregisters a previously
     registered callback. If the number of registered callbacks is
     large, then this can be a slow operation. Generally, the callback
     mechanism is optimized for cases where the callbacks stay
     registered until the transaction is finalized.</p>

  <p>Note also that you don't need to unregister a callback that has
     been called or auto-reset using the <code>state</code> argument
     passed to <code>callback_register()</code>. This function does nothing
     if the key is not found.</p>

  <p>The <code>callback_update()</code> function can be used to update
     the <code>event</code>, <code>data</code>, and <code>state</code>
     values of a previously registered callback. Similar to
     <code>callback_unregister()</code>, this is a potentially slow
     operation.</p>

  <p>When the callback is called, it is passed the event that
     triggered it, as well as the <code>key</code> and
     <code>data</code> values that were passed to the
     <code>callback_register()</code> function. Note also that the order
     in which the callbacks are called is unspecified. The rollback
     event can be triggered by an exception. In this case, if the
     callback throws, the program will be terminated.</p>

  <p>The following example shows how we can use transaction
     callbacks together with database operation callbacks
     (<a href="#14.1.7">Section 14.1.7, "<code>callback</code>"</a>)
     to manage the object's "dirty" flag.</p>

  <pre class="cxx">
#include &lt;odb/callback.hxx>
#include &lt;odb/transaction.hxx>

#pragma db object callback(update)
class object
{
  ...

  #pragma db transient
  mutable bool dirty_;

  // Non-NULL value indicates that we are registered
  // with this transaction.
  //
  #pragma db transient
  mutable odb::transaction* tran_;

  void
  update (odb::callback_event e, odb::database&amp;) const
  {
    using namespace odb::core;

    if (e == callback_event::post_update)
      return;

    // Mark the object as clean again but register a
    // transaction callback in case the update is rolled
    // back.
    //
    tran_ = &amp;transaction::current ();
    tran_->callback_register (&amp;rollback,
                              const_cast&lt;object*> (this),
                              transaction::event_rollback,
                              0,
                              &amp;tran_);
    dirty_ = false;
  }

  static void
  rollback (unsigned short, void* key, unsigned long long)
  {
    // Restore the dirty flag since the changes have been
    // rolled back.
    //
    object&amp; o (*static_cast&lt;object*> (key));
    o.dirty_ = true;
  }

  ~object ()
  {
    // Unregister the callback if we are going away before
    // the transaction.
    //
    if (tran_ != 0)
      tran_->callback_unregister (this);
  }
};
  </pre>

  <h2><a name="15.2">15.2 Persistent Class Template Instantiations</a></h2>

  <p>Similar to composite value types (<a href="#7.2">Section 7.2, "Composite
     Value Types"</a>), a persistent object can be defined as an instantiation
     of a C++ class template, for example:</p>

  <pre class="cxx">
template &lt;typename T>
class person
{
  ...

  T first_;
  T last_;
};

using std_person = person&lt;std::string>;

#pragma db object(std_person)
#pragma db member(std_person::last_) id
  </pre>

  <p>Note that the database support code for such a persistent object
     is generated when compiling the header containing the
     <code>db&nbsp;object</code> pragma and not the header containing
     the template definition or the <code>using</code> alias. This
     allows us to use templates defined in other files, for example:</p>

  <pre class="cxx">
#include &lt;utility> // std::pair

using person = std::pair&lt;unsigned int, std::string>;
#pragma db object(person)
#pragma db member(person::first) id auto column("id")
#pragma db member(person::second) column("name")
  </pre>

  <p>You may also have to explicitly specify the object type in
     calls to certain <code>database</code> class functions due
     to the inability do distinguish, at the API level, between
     smart pointers and persistent objects defined as class
     template instantiations. For example:</p>

  <pre class="cxx">
person p;

db.update (p); // Error.
db.reload (p); // Error.
db.erase (p);  // Error.

db.update&lt;person> (p); // Ok.
db.reload&lt;person> (p); // Ok.
db.erase&lt;person> (p);  // Ok.
  </pre>

  <p>It also makes sense to factor persistent data members that do not
     depend on template arguments into a common, non-template base class.
     The following more realistic example illustrates this approach:</p>

  <pre class="cxx">
#pragma db object abstract
class base_common
{
  ...

  #pragma db id auto
  unsigned long long id;
};

template &lt;typename T>
class base: public base_common
{
  ...

  T value;
};

using string_base = base&lt;std::string>;
#pragma db object(string_base) abstract

#pragma db object
class derived: public string_base
{
  ...
};
  </pre>

  <h2><a name="15.3">15.3 Bulk Database Operations</a></h2>

  <p>Some database systems supported by ODB provide a mechanism, often called
     bulk or batch statement execution, that allows us to execute the same SQL
     statement on multiple sets of data at once and with a single database API
     call (or equivalent). This often results in significantly better
     performance if we need to execute the same statement for a large number
     of data sets (thousands to millions).</p>

  <p>ODB translates this mechanism to bulk operations which allow us to
     persist, update, or erase a range of objects in the database. Currently,
     from all the database systems supported by ODB, only Oracle, Microsoft
     SQL Server, and PostgreSQL are capable of bulk operations (but
     see <a href="#19.5.7">Section 19.5.7, "Bulk Operations Support"</a> for
     PostgreSQL limitations). There is also currently no emulation of the bulk
     API for other databases nor dynamic multi-database support. As a result,
     if you are using dynamic multi-database support, you will need to "drop
     down" to static support in order to access the bulk API.  Refer
     to <a href="#16">Chapter 16, "Multi-Database Support"</a> for
     details.</p>

  <p>As we will discuss later in this section, bulk operations have
     complex failure semantics that is dictated by the underlying
     database API. As a result, support for bulk persist, update,
     and erase is limited to persistent classes for which these
     operations can be performed with a single database statement
     execution. In particular, bulk operations are not available
     for polymorphic objects (<a href="#8.2">Section 8.2,
     "Polymorphism Inheritance"</a>) or objects that have
     containers (inverse containers of object pointers are an
     exception). Furthermore, for objects that have sections
     (<a href="#9">Chapter 9, "Sections"</a>) the bulk update operation
     will only be available if all the sections are manually-updated.
     On the other hand, bulk operations are supported for objects
     that use optimistic concurrency (<a href="#12">Chapter 12,
     "Optimistic Concurrency"</a>) or have no object id
     (<a href="#14.1.6">Section 14.1.6, "<code>no_id</code>"</a>).</p>

  <p>To enable the generation of bulk operation support for a persistent
     class we use the <code>bulk</code> pragma. For example:</p>

  <pre class="cxx">
#pragma db object bulk(5000)
class person
{
  ...

  #pragma db id auto
  unsigned long long id;
};
  </pre>

  <p>The single argument to the <code>bulk</code> pragma is the batch
     size. The batch size specifies the maximum number of data sets that
     should be handled with a single underlying statement execution (or
     equivalent). If the range that we want to perform the bulk operation on
     contains more objects than the batch size, then ODB will split this
     operation into multiple underlying statement executions (batches). To
     illustrate this point with an example, suppose we want to persist 53,000
     objects and the batch size is 5,000. ODB will then execute the statement
     11 times, the first 10 times with 5,000 data sets each, and the last time
     with the remaining 3,000 data sets.</p>

  <p>The commonly used batch sizes are in the 2,000-5,000 range, though
     smaller or larger batches could provide better performance,
     depending on the situation. As a result, it is recommended to
     experiment with different batch sizes to determine the optimum
     number for a particular object and its use-cases. Note also that
     you may achieve better performance by also splitting a large bulk
     operation into multiple transactions (<a href="#3.5">Section 3.5,
     "Transactions"</a>).</p>

  <p>For database systems that do not support bulk operations the
     <code>bulk</code> pragma is ignored. It is also possible to
     specify different batch sizes for different database systems
     by using the database prefix, for example:</p>

  <pre class="cxx">
#pragma db object mssql:bulk(3000) oracle:bulk(4000) pgsql:bulk(2000)
class person
{
  ...
};
  </pre>

  <p>Note that while specifying the batch size at compile time might
     seem inflexible, this approach allows ODB to place internal
     arrays of the fixed batch size on the stack rather than
     allocating them in the dynamic memory. However, specifying the
     batch size at runtime may be supported in the future.</p>

  <p>Once the bulk support is enabled for a particular object, we can
     use the following <code>database</code> functions to perform bulk
     operations:</p>

  <pre class="cxx">
template &lt;typename I>
void
persist (I begin, I end, bool continue_failed = true);

template &lt;typename I>
void
update (I begin, I end, bool continue_failed = true);

template &lt;typename I>
void
erase (I obj_begin, I obj_end, bool continue_failed = true);

template &lt;typename T, typename I>
void
erase (I id_begin, I id_end, bool continue_failed = true);
  </pre>

  <p>Every bulk API function expects a range of elements, passed in
     the canonical C++ form as a pair of input iterators. In case of
     <code>persist()</code>, <code>update()</code>, and the first
     <code>erase()</code> overload, we pass a range of objects,
     either as references or as pointers, raw or smart. The following
     example illustrates the most common scenarios using the
     <code>persist()</code> call:</p>

  <pre class="cxx">
// C array of objects.
//
person a[2] {{"John", "Doe"}, {"Jane", "Doe"}};

db.persist (a, a + sizeof(a) / sizeof(a[0]));


// Vector of objects.
//
std::vector&lt;person> v {{"John", "Doe"}, {"Jane", "Doe"}};

db.persist (v.begin (), v.end ());


// C array of raw pointers to objects.
//
person p1 ("John", "Doe");
person p2 ("Jane", "Doe");
person* pa[2] {&amp;p1, &amp;p2};

db.persist (pa, pa + sizeof(pa) / sizeof(pa[0]));


// Vector of raw pointers to objects.
//
std::vector&lt;person*> pv {&amp;p1, &amp;p2};

db.persist (pv.begin (), pv.end ());


// Vector of smart (shared) pointers to objects.
//
std::vector&lt;std::shared_ptr&lt;person>> sv {
  std::make_shared&lt;person> ("John", "Doe"),
  std::make_shared&lt;person> ("Jane", "Doe")};

db.persist (sv.begin (), sv.end ());
  </pre>

  <p>The ability to perform a bulk operation on a range of raw pointers
     to objects can be especially useful when the application stores
     objects in a way that does not easily conform to the pair of
     iterators interface. In such cases we can create a temporary
     container of shallow pointers to objects and use that to perform
     the bulk operation, for example:</p>

  <pre class="cxx">
struct person_entry
{
  person obj;

  // Some additional data.
  ...
};

using people = std::vector&lt;person_entry>;

void
persist (odb::database&amp; db, people&amp; p)
{
  std::vector&lt;person*> tmp;
  tmp.reserve (p.size ());
  std::for_each (p.begin (),
                 p.end (),
                 [&amp;tmp] (person_entry&amp; pe)
                 {
                   tmp.push_back (&amp;pe.obj);
                 });


  db.persist (tmp.begin (), tmp.end ());
}
  </pre>

  <p>The second overload of the bulk <code>erase()</code> function
     allows us to pass a range of object ids rather than objects
     themselves. As with the corresponding non-bulk version, we
     have to specify the object type explicitly, for example:</p>

  <pre class="cxx">
std::vector&lt;unsigned long long> ids {1, 2};

db.erase&lt;person> (ids.begin (), ids.end ());
  </pre>

  <p>Conceptually, a bulk operation is equivalent to performing the
     corresponding non-bulk version in a loop, except when it comes to the
     failure semantics. Some databases that currently are capable of bulk
     operations (specifically, Oracle and SQL Server) do not stop when a data
     set in a batch fails (for example, because of a unique constraint
     violation). Instead, they continue executing subsequent data sets until
     every element in the batch has been attempted. The
     <code>continue_failed</code> argument in the bulk functions listed
     above specifies whether ODB should extend this behavior and continue
     with subsequent batches if the one it has tried to execute has failed
     elements. The default behavior is to continue.</p>

  <p>The consequence of this failure semantics is that we may have
     multiple elements in the range failed for different reasons.
     For example, if we tried to persist a number of objects, some
     of them might have failed because they are already persistent
     while others &mdash; because of a unique constraint violation.
     As a result, ODB uses the special <code>odb::multiple_exceptions</code>
     class to report failures in the bulk API functions. This
     exception is thrown if one or more elements in the range have
     failed and it contains the error information in the form of other
     ODB exception for each failed position. The
     <code>multiple_exceptions</code> class has the following interface:</p>

  <pre class="cxx">
struct multiple_exceptions: odb::exception
{
  // Element type.
  //
  struct value_type
  {
    std::size_t
    position () const;

    const odb::exception&amp;
    exception () const;

    bool
    maybe () const;
  };

  // Iteration.
  //
  using set_type = std::set&lt;value_type>;

  using iterator = set_type::const_iterator;
  using const_iterator = set_type::const_iterator;

  iterator
  begin () const;

  iterator
  end () const;

  // Lookup.
  //
  const value_type*
  operator[] (std::size_t) const;

  // Severity, failed and attempted counts.
  //
  std::size_t
  attempted () const;

  std::size_t
  failed () const;

  bool
  fatal () const;

  void
  fatal (bool);

  // Direct data access.
  //
  const set_type&amp;
  set () const;

  // odb::exception interface.
  //
  virtual const char*
  what () const throw ();
};
  </pre>

  <p>The <code>multiple_exceptions</code> class has a map-like interface
     with the key being the position in the range and the value being
     the exception plus the <code>maybe</code> flag (discussed below).
     As a result, we can either iterate over the failed positions or
     we can check whether a specific position in the range has failed.
     The following example shows what a <code>catch</code>-handler for
     this exception might look like:</p>

  <pre class="cxx">
std::vector&lt;person> objs {{"John", "Doe"}, {"Jane", "Doe"}};

try
{
  db.persist (objs.begin (), objs.end ());
}
catch (const odb::multiple_exceptions&amp; me)
{
  for (const auto&amp; v: me)
  {
    size_t p (v.position ());

    try
    {
      throw v.exception ();
    }
    catch (const odb::object_already_persistent&amp;)
    {
      cerr &lt;&lt; p &lt;&lt; ": duplicate id: " &lt;&lt; objs[p].id () &lt;&lt; endl;
    }
    catch (const odb::exception&amp; e)
    {
      cerr &lt;&lt; p &lt;&lt; ": " &lt;&lt; e.what () &lt;&lt; endl;
    }
  }
}
  </pre>

  <p>If, however, all we want is to show the diagnostics to the user,
     then the string returned by the <code>what()</code> function
     will contain the error information for each failed position.
     Here is what it might look like (using Oracle as an example):</p>

  <pre class="terminal">
multiple exceptions, 4 elements attempted, 2 failed:
[0] object already persistent
[3] 1: ORA-00001: unique constraint (ODB_TEST.person_last_i) violated
  </pre>

  <p>Some databases that currently are capable of bulk operations
     (specifically, Oracle and SQL Server) return a total count of affected
     rows rather than individual counts for each data set. This limitation
     prevents ODB from being able to always determine which elements in the
     batch haven't affected any rows and, for the update and erase operations,
     translate this to the <code>object_not_persistent</code> exceptions. As a
     result, if some elements in the batch haven't affected any rows and ODB
     is unable to determine exactly which ones, it will mark all the elements
     in this batch as "maybe not persistent". That is, it will insert
     the <code>object_not_persistent</code> exception and set
     the <code>maybe</code> flag for every position in the batch. The
     diagnostics string returned by <code>what()</code> will also reflect this
     situation, for example (assuming batch size of 3):</p>

  <pre class="terminal">
multiple exceptions, 4 elements attempted, 4 failed:
[0-2] (some) object not persistent
[3] object not persistent
  </pre>

  <p>The way to handle and recover from such "maybe failures" will have
     to be application-specific. For example, for some applications the
     fact that some objects no longer exist in the database when
     performing bulk erase might be an ignorable error. If, however,
     the application needs to determine exactly which elements in the batch
     have failed, then a <code>load()</code> call will be required for each
     element in the batch (or a query using a view to avoid loading all
     the data members; <a href="#10">Chapter 10, "Views"</a>). This is also
     something to keep in mind when selecting the batch size since for
     larger sizes it will be more expensive (more loads to perform) to
     handle such "maybe failures". If the failures are not uncommon, as
     is the case, for example, when using optimistic concurrency, then
     it may make sense to use a smaller batch.</p>

  <p>The lookup operator (<code>operator[]</code>) returns <code>NULL</code>
     if the element at this position has no exception. Note also that the
     returned value is <code>value_type*</code> and not
     <code>odb::exception*</code> in order to provide access to the
     <code>maybe</code> flag discussed above.</p>

  <p>The <code>multiple_exceptions</code> class also provides access
     to the number of positions attempted (the <code>attempted()</code>
     accessor) and failed (the <code>failed()</code> accessor). Note
     that the failed count includes the "maybe failed" positions.</p>

  <p>The <code>multiple_exceptions</code> exception can also be fatal.
     If the <code>fatal()</code> accessor returns <code>true</code>, then
     (some of) the exceptions were fatal. In this case, even for positions
     that did not fail, no attempts were made to complete the operation
     and the transaction must be aborted.</p>

  <p>If <code>fatal()</code> returns false, then the operation on the
     elements that don't have an exception has succeeded. The application
     can ignore the errors or try to correct the errors and re-attempt
     the operation on the elements that did fail. In either case, the
     transaction can be committed.</p>

  <p>An example of a fatal exception would be the situation where the
     execution of the underlying statement failed summarily, without
     attempting any data sets, for instance, because of an error in
     the statement itself.</p>

  <p>The <code>fatal()</code> modifier allows you to "upgrade" an
     exception to fatal, for example, for specific database error
     codes.</p>


  <!-- PART -->


  <hr class="page-break"/>
  <h1><a name="II">PART II&nbsp;&nbsp;
      <span style="font-weight: normal;">DATABASE SYSTEMS</span></a></h1>

  <p>Part II covers topics specific to the database system
     implementations and their support in ODB. The first chapter in
     Part II discusses how to use multiple database systems in the
     same application. The subsequent chapters describe the system-specific
     <code>database</code> classes as well as the default mapping
     between basic C++ value types and native database types. Part
     II consists of the following chapters.</p>

  <table class="toc">
    <tr><th>16</th><td><a href="#16">Multi-Database Support</a></td></tr>
    <tr><th>17</th><td><a href="#17">MySQL Database</a></td></tr>
    <tr><th>18</th><td><a href="#18">SQLite Database</a></td></tr>
    <tr><th>19</th><td><a href="#19">PostgreSQL Database</a></td></tr>
    <tr><th>20</th><td><a href="#20">Oracle Database</a></td></tr>
    <tr><th>21</th><td><a href="#21">Microsoft SQL Server Database</a></td></tr>
  </table>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="16">16 Multi-Database Support</a></h1>

  <p>Some applications may need to access multiple database systems, either
     simultaneously or one at a time. For example, an application may
     utilize an embedded database such as SQLite as a local cache and use
     a client-server database such as PostgreSQL for more permanent
     but slower to access remote storage. Or an application may need
     to be able to store its data in any database selected at runtime
     by the user. Yet another scenario is the data migration from one
     database system to another. In this case, multi-database support
     is only required for a short period. It is also plausible that an
     application implements all three of these scenarios, that is, it
     uses SQLite as a local cache, allows the user to select the remote
     database system, and supports data migration from one remote database
     system to another.</p>

  <p>ODB provides two types of multi-database support: <em>static</em>
     and <em>dynamic</em>. With static support we use the
     database system-specific interfaces to perform database
     operations. That is, instead of using <code>odb::database</code>,
     <code>odb::transaction</code>, or <code>odb::query</code>, we
     would use, for example, <code>odb::sqlite::database</code>,
     <code>odb::sqlite::transaction</code>, or
     <code>odb::sqlite::query</code> to access an SQLite database.</p>

  <p>In contrast, with <em>dynamic</em> multi-database support we can
     use the common interface to access any database without having to
     know which one it is. At runtime, ODB will automatically dispatch
     a call on the common interface to the specific database implementation
     based on the actual <code>database</code> instance being
     used. In fact, this mechanism is very similar to C++ virtual
     functions.</p>

  <p>Both static and dynamic multi-database support have a different set
     of advantages and disadvantages which makes them more or less suitable
     for different use cases. Static support has zero overhead compared
     to single-database support and allows us to use database
     system-specific features, extensions, etc. At the same time, the
     code that we write will be tied to the specific database system.
     As a result, this type of multi-database support is more
     suitable for situations where different parts of an application
     access different but specific database systems. For example,
     using SQLite as a local cache most likely falls into this
     category since we are using a specific database system (SQLite)
     and the code that will check the cache will most likely (but
     not necessarily) be separate from the code that interact with
     the remote database. Another example where static multi-database
     support might be more suitable is a once-off data migration from
     one database system to another. In this case both the source and
     target are specific database systems. In contrast, if data migration
     from one database system to another is a general feature in an
     application, then dynamic multi-database support might be more
     suitable.</p>

  <p>The main advantage of dynamic multi-database support is the
     database system-independence of the code that we write. The same
     application code will work with any database system supported by
     ODB and the generated database support code can be packaged into
     separate libraries and loaded dynamically by the application. The
     disadvantages of dynamic support are slight overhead and certain
     limitations in functionality compared to static support (see
     <a href="#16.2">Section 16.2, "Dynamic Multi-Database Support"</a>
     for details). As a result, dynamic multi-database support is most
     suitable to situations where we need the same code to
     work with a range of database systems. For example, if your
     application must be able to store its data in any database
     selected by the user, then dynamic support is probably the
     best option.</p>

  <p>Note also that it is possible to mix and match static and dynamic
     support in the same application. In fact, dynamic support is built
     on top of static support so it is possible to use the same database
     system both "statically" and "dynamically". In particular, the ability
     to "drop down" from dynamic to static support can be used to overcome
     the functionality limitations mentioned above. Finally,
     single-database support is just a special case of static
     multi-database support with a single database system.</p>

  <p>By default ODB assumes single-database support. To enable
     multi-database support we use the <code>--multi-database</code>
     (or <code>-m</code>) ODB compiler option. This option is also used to
     specify the support type: <code>static</code> or <code>dynamic</code>.
     For example:</p>

  <pre class="terminal">
odb -m static ... person.hxx
  </pre>

  <p>With multi-database support enabled, we can now generate the database
     support code for several database systems. This can be accomplished
     either with a single ODB compiler invocation by specifying multiple
     <code>--database</code> (or <code>-d</code>) options or with multiple
     ODB compiler invocations. Both approaches produce the same result,
     for example:</p>

  <pre class="terminal">
odb -m static -d common -d sqlite -d pgsql person.hxx
  </pre>

  <p>Is equivalent to:</p>

  <pre class="terminal">
odb -m static -d common person.hxx
odb -m static -d sqlite person.hxx
odb -m static -d pgsql person.hxx
  </pre>

  <p>Notice that the first <code>-d</code> option has <code>common</code>
     as its value. This is not a real database system. Rather, it instructs
     the ODB compiler to generate code that is common to all the database
     systems and, in case of dynamic support, is also the common
     interfaces.</p>

  <p>If you look at the result of the above commands, you will also notice
     changes in the output file names. In the single-database mode the ODB
     compiler produces a single set of the <code>person-odb.?xx</code> files
     which contain both the common as well as the database specific
     generated code (since there is only one database system in use,
     there is no reason to split the two). In contrast, in the
     multi-database mode, the <code>person-odb.?xx</code> set of files
     contains the common code while the database system-specific code is
     written to files in the form <code>person-odb-&lt;db>.?xx</code>.
     That is, <code>person-odb-sqlite.?xx</code> for SQLite,
     <code>person-odb-pgsql.?xx</code> for PostgreSQL, etc.</p>

  <p>If we need dynamic support for some databases and static for
     others, then the <code>common</code> code must be generated
     in the dynamic mode. For example, if we need static support
     for SQLite and dynamic support for PostgreSQL and Oracle, then
     the ODB compiler invocations could look like this:</p>

  <pre class="terminal">
odb -m dynamic -d common person.hxx
odb -m static -d sqlite person.hxx
odb -m dynamic -d pgsql person.hxx
odb -m dynamic -d oracle person.hxx
  </pre>

  <p>With multi-database support enabled, it is possible to restrict ODB
     pragmas to apply only to a specific database system (unrestricted
     pragmas apply to all the databases). For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db pgsql:type("VARCHAR(128)") sqlite:type("TEXT")
  std::string name_;

  unsigned short age_;

  #pragma db pgsql index member(age_)
};
  </pre>

  <p>Above, the pragma for the <code>name_</code> data member shows the
     use of a database prefix (for example, <code>pgsql:</code>) that
     only applies to the specifier that follows. The pragma that defines
     an index on the <code>age_</code> data member shows the use of a
     database prefix that applies to the whole pragma. In this case the
     database name must immediately follow the <code>db</code> keyword.</p>


  <p>Similar to pragmas, ODB compiler options that determine the kind
     (for example, <code>--schema-format</code>), names (for example,
     <code>--odb-file-suffix</code>), or content (for example, prologue
     and epilogue options) of the output files can be prefixed with the
     database name. For example:</p>

  <pre class="terminal">
odb --odb-file-suffix common:-odb-common ...
  </pre>

  <p>Dynamic multi-database support requires consistent mapping across
     all the databases. That is, the same classes and data members
     should be mapped to objects, simple/composite values, etc., for
     all the databases. In contrast, static multi-database support
     does not have this restriction. Specifically, with static support,
     some data members can be transient for some database systems.
     Similarly, the same class (for example, <code>point</code>) can
     be mapped to a simple value in one database (for example, to the
     <code>POINT</code> PostgreSQL type) and to a composite value
     in another (for example, in SQLite, which does not have a
     built-in point type).</p>

  <p>The following sections discuss static and dynamic multi-database
     support in more detail.</p>


  <h2><a name="16.1">16.1 Static Multi-Database Support</a></h2>

  <p>With static multi-database support, instead of including
     <code>person-odb.hxx</code>, application source code has
     to include <code>person-odb-&lt;db>.hxx</code> header files
     corresponding to the database systems that will be used.</p>

  <p>The application code has to also use database system-specific
     interfaces when performing database operations. As an example,
     consider the following transaction in a single-database
     application. It uses the common interfaces, that is, classes
     from the <code>odb</code> namespace.</p>

  <pre class="cxx">
#include "person-odb.hxx"

odb::database&amp; db = ...

using query = odb::query&lt;person>;
using result = odb::result&lt;person>;

odb::transaction t (db.begin ());
result r (db.query&lt;person> (query::age &lt; 30));
...
t.commit ();
  </pre>

  <p>In an application that employs static multi-database support
     the same transaction for SQLite would be rewritten like this:</p>

  <pre class="cxx">
#include "person-odb-sqlite.hxx"

odb::sqlite::database&amp; db = ...

using query = odb::sqlite::query&lt;person>;
using result = odb::result&lt;person>;      // odb:: not odb::sqlite::

odb::sqlite::transaction t (db.begin ());
result r (db.query&lt;person> (query::age &lt; 30));
...
t.commit ();
  </pre>

  <p>That is, the <code>database</code>, <code>transaction</code>, and
     <code>query</code> classes now come from the <code>odb::sqlite</code>
     namespace instead of <code>odb</code>. Other classes that have
     database system-specific interfaces are <code>connection</code>,
     <code>statement</code>, and <code>tracer</code>. Note that
     all of them derive from the corresponding common versions. It
     is also possible to use common <code>transaction</code>,
     <code>connection</code>, and <code>statement</code> classes
     with static support, if desired.</p>

  <p>Notice that we didn't use the <code>odb::sqlite</code> namespace
     for the <code>result</code> class template. This is because
     <code>result</code> is database system-independent. All other
     classes defined in namespace <code>odb</code>, except those
     specifically mentioned above, are database system-independent.
     In particular, <code>result</code>, <code>prepared_query</code>,
     <code>session</code>, <code>schema_catalog</code>, and all the
     exceptions are database system-independent.</p>

  <p>Writing <code>odb::sqlite::</code> before every name can quickly
     become burdensome. As we have seen before, in single-database
     applications that use the common interface we can add the
     <code>using namespace</code> directive to avoid qualifying
     each name. For example:</p>

  <pre class="cxx">
#include "person-odb.hxx"

odb::database&amp; db = ...

{
  using namespace odb::core;

  using person_query = query&lt;person>;
  using person_result = result&lt;person>;

  transaction t (db.begin ());
  person_result r (db.query&lt;person> (person_query::age &lt; 30));
  ...
  t.commit ();
}
  </pre>

  <p>A similar mechanism is available in multi-database support. Each
     database runtime defines the <code>odb::&lt;db>::core</code>
     namespace that contains all the database system-independent
     names as well as the database system-specific ones for this
     database. Here is how we can rewire the above transaction
     using this approach:</p>

  <pre class="cxx">
#include "person-odb-sqlite.hxx"

odb::sqlite::database&amp; db = ...

{
  using namespace odb::sqlite::core;

  using person_query = query&lt;person>;
  using person_result = result&lt;person>;

  transaction t (db.begin ());
  person_result r (db.query&lt;person> (person_query::age &lt; 30));
  ...
  t.commit ();
}
  </pre>

  <p>If the <code>using namespace</code> directive cannot be used, for
     example, because the same code fragment accesses several databases,
     then we can still make the namespace qualifications more concise
     by assigning shorter aliases to database namespaces. For example:</p>

  <pre class="cxx">
#include "person-odb-pgsql.hxx"
#include "person-odb-sqlite.hxx"

namespace pg = odb::pgsql;
namespace sl = odb::sqlite;

pg::database&amp; pg_db = ...
sl::database&amp; sl_db = ...

using pg_query = pg::query&lt;person>;
using sl_query = sl::query&lt;person>;
using result = odb::result&lt;person>;

// First check the local cache.
//
odb::transaction t (sl_db.begin ()); // Note: using common transaction.
result r (sl_db.query&lt;person> (sl_query::age &lt; 30));

// If no hits, try the remote database.
//
if (r.empty ())
{
  t.commit ();              // End the SQLite transaction.
  t.reset (pg_db.begin ()); // Start the PostgreSQL transaction.

  r = pg_db.query&lt;person> (pg_query::age &lt; 30);
}

// Handle the result.
//
...

t.commit ();
  </pre>

  <p>With static multi-database support we can make one of the databases
     the default database with the <code>--default-database</code> option.
     The default database can be accessed via the common interface, just
     like with single-database support. For example:</p>

  <pre class="terminal">
odb -m static -d common -d pgsql -d sqlite --default-database pgsql ...
  </pre>

  <p>The default database mechanism can be useful when one of the
     databases is primary or when retrofitting multi-database support
     into an existing single-database application. For example, if
     we are adding SQLite as a local cache into an existing
     application that uses PostgreSQL as its only database, then
     by making PostgreSQL the default database we avoid having to
     change all the existing code. Note that if dynamic multi-database
     support is enabled, then the common (dynamic) interface is always
     made the default database.</p>

  <h2><a name="16.2">16.2 Dynamic Multi-Database Support</a></h2>

  <p>With dynamic multi-database support, application source code only
     needs to include the <code>person-odb.hxx</code> header file, just
     like with single-database support. In particular, we don't need
     to include any of the <code>person-odb-&lt;db>.hxx</code> files
     unless we would also like to use certain database systems in the
     static multi-database mode.</p>

  <p>When performing database operations, the application code
     uses the common interfaces from the <code>odb</code> namespace,
     just like with single-database support. As an example, consider
     a function that can be used to load an object either from a local
     SQLite cache or a remote PostgreSQL database (in reality, this
     function can be used with any database system support by ODB
     provided we generated the database support code for this database
     and linked it into our application):</p>

  <pre class="cxx">
#include "person-odb.hxx"

std::unique_ptr&lt;person>
load (odb::database&amp; db, const std::string&amp; name)
{
  odb::transaction t (db.begin ());
  std::unique_ptr&lt;person> p (db.find (name));
  t.commit ();
  return p;
}

odb::pgsql::database&amp; pg_db = ...
odb::sqlite::database&amp; sl_db = ...

// First try the local cache.
//
std::unique_ptr&lt;person> p (load (sl_db, "John Doe"));

// If not found, try the remote database.
//
if (p == 0)
  p = load (pg_db, "John Doe");

...
  </pre>

  <p>As you can see, we can use dynamic multi-database support just like
     single-database support except that now our code can work with
     different database systems. Note, however, one difference: with
     single-database support we could perform database operations using
     either the common <code>odb::database</code> or a database system-specific
     (for example, <code>odb::sqlite::database</code>) interface
     with the same effect. In contrast, with dynamic multi-database support,
     the use of the database system-specific interface results in the
     switch to the static mode (for which, as was mentioned earlier, we would
     need to include the corresponding <code>person-odb-&lt;db>.hxx</code>
     header file). As we will discuss shortly, switching from dynamic to
     static mode can be used to overcome limitations imposed by dynamic
     multi-database support.</p>

  <p>Dynamic multi-database support has certain overheads and limitations
     compared to static support. For database operations, the generated code
     maintains function tables that are used to dispatch calls to the database
     system-specific implementations. In single-database and static
     multi-database support, the <code>query</code> type implements a thin
     wrapper around the underlying database system's <code>SELECT</code>
     statement. With dynamic multi-database support, because the
     underlying database system is only known at query execution
     (or preparation) time, the <code>query</code> type stores a
     database system-independent representation of the query that
     is then translated to the database system-specific form. Because
     of this database system-independent representation, dynamic
     support queries have a number of limitations. Specifically, dynamic
     queries do not support parameter binding in native query fragments.
     They also make copies of by-value parameterd (by-reference parameters
     can be used to remove this overhead). Finally, parameters of array
     types (for example, <code>char[256]</code>) can only be bound
     by-reference.</p>

  <p>As we mentioned earlier, switching from dynamic to static mode
     can be an effective way to overcome these limitations. As an
     example, consider a function that prints the list of people of
     a certain age. The caller also specified the limit on the number
     of entries to print. Some database systems, for example, PostgreSQL,
     allow us to propagate this limit to the database server with the
     <code>LIMIT</code> clause. To add this clause we would need to
     construct a native query fragment and, as we discussed above, we
     won't be able to bind a parameter (the limit) while in the dynamic
     mode. The following implementation shows how we can overcome this
     by switching to the static mode and using the PostgreSQL-specific
     interface:</p>

  <pre class="cxx">
#include "person-odb.hxx"
#include "person-odb-pgsql.hxx" // Needed for static mode.

void
print (odb::database&amp; db, unsigned short age, unsigned long limit)
{
  using query = odb::query&lt;person>;
  using result = odb::result&lt;person>;

  odb::transaction t (db.begin ());

  query q (query::age == age);
  result r;

  if (db.id () == odb::id_pgsql)
  {
    // We are using PostgreSQL. Drop down to the static mode and
    // add the LIMIT clause to the query.
    //
    namespace pg = odb::pgsql;
    using pg_query = pg::query&lt;person>;

    pg::database&amp; pg_db (static_cast&lt;pg::database&amp;> (db));
    pg_query pg_q (pg_query (q) + "LIMIT" + pg_query::_val (limit));
    r = pg_db.query&lt;person> (pg_q);
  }
  else
    r = db.query&lt;person> (q);

  // Handle the result up to the limit elements.
  //
  ...

  t.commit ();
}

odb::pgsql::database&amp; pg_db = ...
odb::sqlite::database&amp; sl_db = ...

print (sl_db, 30, 100);
print (sl_db, 30, 100);
  </pre>

  <p>A few things to note about this example. First, we use the
     <code>database::id()</code> function to determine the actual database
     system we use. This function has the following signature:</p>

  <pre class="cxx">
namespace odb
{
  enum database_id
  {
    id_mysql,
    id_sqlite,
    id_pgsql,
    id_oracle,
    id_mssql,
    id_common
  };

  class database
  {
  public:
    ...

    database_id
    id () const;
  }
}
  </pre>

  <p>Note that <code>database::id()</code> can never return the
     <code>id_common</code> value.</p>

  <p>The other thing to note is how we translate the dynamic query
     to the database system-specific one (the <code>pg_query (q)</code>
     expression). Every <code>odb::&lt;db>::query</code> class provides
     such a translation constructor.</p>

  <h3><a name="16.2.2">16.2.2 Dynamic Loading of Database Support Code</a></h3>

  <p>With dynamic multi-database support, the generated database support
     code automatically registers itself with the function tables that
     we mentioned earlier. This makes it possible to package the generated
     code for each database into a separate dynamic-link library (Windows
     DLL) or dynamic shared object (Unix DSO; collectively referred to as
     DLLs from now on) and load/unload them from the application
     dynamically using APIs such as Win32 <code>LoadLibrary()</code> or
     POSIX <code>dlopen()</code>. This allows the application address
     space to contain code only for database systems that are actually
     needed in any particular moment. Another advantage of this approach
     is the ability to distribute individual database system support
     separately.</p>

  <p>This section provides an overview of how to package the generated
     database support code into DLLs for both Windows and Unix using
     GNU/Linux as an example. Note also that if static multi-database
     support is used for a particular database system, then the dynamic
     loading cannot be used for this database. It is, however, still
     possible to package the generated code into a DLL but this DLL
     will have to be linked to the executable at link-time rather
     than at runtime. If dynamic loading is desirable in this situation,
     then another alternative would be to package the functionality
     that requires static support together with the database support
     code into the DLL and import this functionality dynamically
     using the <code>GetProcAddress()</code> (Win32) or <code>dlsym()</code>
     (Unix) function.</p>

  <p>The first step in packaging the generated code into DLLs is to
     set up the symbol exporting. This step is required for
     Windows DLLs but is optional for Unix DSOs. Most modern Unix
     systems (such as GNU/Linux) provide control over symbol
     visibility, which is a mechanism similar to Windows symbol
     exporting. Notable advantages of using this mechanism to
     explicitly specify which symbols are visible include
     smaller Unix DSOs and faster load times. If, however, you are
     not planning to control symbol visibility on Unix, then you can
     skip directly to the second step below.</p>

  <p>An important point to understand is that we only need to export
     the common interface, that is, the classes defined in the
     <code>person-odb.hxx</code> header. In particular, we don't need
     to export the database system-specific classes defined in
     the <code>person-odb-&lt;db>.hxx</code>, unless we are also using
     this database in the static mode (in which case, the procedure
     described below will need to be repeated for that database as
     well).</p>

  <p>The ODB compiler provides two command line options,
     <code>--export-symbol</code> and <code>--extern-symbol</code>,
     which can be used to insert the export and extern
     macros in all the necessary places in the generated header file.
     You are probably familiar with the concept of export macro which
     expands to an export directive if we are building the DLL and to
     an import directive if we are building client code. The
     extern macro is a supplementary mechanism which is necessary to
     export explicit template instantiations used by the generated
     code when query support is enabled. As we will see shortly, the
     extern macro must expand into the <code>extern</code> C++ keyword
     in certain situations and must be left undefined in others. To
     manage all these macro definitions, it is customary to create the
     so called export header. Based on a single macro that is normally
     defined in the project file or on the command line and which
     indicates whether we are building the DLL or client code, the
     export header file sets the export and extern macros to their
     appropriate values. Continuing with our person example, on Windows
     the export header, which we will call <code>person-export.hxx</code>,
     could look like this:</p>

  <pre class="cxx">
// person-export.hxx
//
// Define PERSON_BUILD_DLL if we are building the DLL. Leave it
// undefined in client code.
//
#ifndef PERSON_EXPORT_HXX
#define PERSON_EXPORT_HXX

#ifdef PERSON_BUILD_DLL
#  define PERSON_EXPORT __declspec(dllexport)
#else
#  define PERSON_EXPORT __declspec(dllimport)
#  define PERSON_EXTERN extern
#endif

#endif // PERSON_EXPORT_HXX
  </pre>

  <p>The equivalent export header for GCC on GNU/Linux is shown below.
     Note also that on GNU/Linux, by default, all symbols are visible
     and we need to add the GCC <code>-fvisibility=hidden</code> option to
     make them hidden by default.</p>

  <pre class="cxx">
// person-export.hxx
//
#ifndef PERSON_EXPORT_HXX
#define PERSON_EXPORT_HXX

#define PERSON_EXPORT __attribute__ ((visibility ("default")))
#define PERSON_EXTERN extern

#endif // PERSON_EXPORT_HXX
  </pre>

  <p>Next we need to export the <code>person</code> persistent class
     using the export macro and re-compile our <code>person.hxx</code> file
     with the <code>--export-symbol</code> and <code>--extern-symbol</code>
     options. We will also need to include <code>person-export.hxx</code>
     into the generated <code>person-odb.hxx</code> file. For that we use
     the <code>--hxx-prologue</code> option. Here is how we can do
     this with multiple invocations of the ODB compiler:</p>

  <pre class="terminal">
odb -m dynamic -d common --hxx-prologue "#include \"person-export.hxx\"" \
--export-symbol PERSON_EXPORT --extern-symbol PERSON_EXTERN person.hxx

odb -m dynamic -d sqlite person.hxx
odb -m dynamic -d pgsql person.hxx
  </pre>

  <p>It is also possible to achieve the same with a single invocation.
     Here we need to restrict some option values to apply only to the
     <code>common</code> database:</p>

  <pre class="terminal">
odb -m dynamic -d common -d sqlite -d pgsql \
--hxx-prologue "common:#include \"person-export.hxx\"" \
--export-symbol common:PERSON_EXPORT --extern-symbol common:PERSON_EXTERN \
person.hxx
  </pre>

  <p>The second step in packaging the generated code into DLLs is to
     decide where to place the generated common interface code. One
     option is to place it into a DLL of its own so that we will end
     up with (replace <code>*.dll</code> with <code>lib*.so</code> for
     Unix): <code>person.dll</code> plus <code>person-sqlite.dll</code> and
     <code>person-pgsql.dll</code>, which both link to <code>person.dll</code>,
     as well as <code>person.exe</code>, which links to <code>person.dll</code>
     and dynamically loads <code>person-sqlite.dll</code>
     and/or <code>person-pgsql.dll</code>. If this is the organization
     that you prefer, then the next step is to build all the DLLs as you
     normally would any other DLL, placing <code>person-odb.cxx</code>
     and <code>person.cxx</code> into <code>person.dll</code>,
     <code>person-odb-sqlite.cxx</code> into <code>person-sqlite.dll</code>,
     etc. Note that in the pure dynamic multi-database support,
     <code>person-sqlite.dll</code> and <code>person-pgsql.dll</code>
     do not export any symbols.</p>

  <p>We can improve on the above organization by getting rid of
     <code>person.dll</code>, which is not really necessary unless
     we have multiple executables sharing the same database support.
     To achieve this, we will place <code>person-odb.cxx</code> into
     <code>person.exe</code> and export its symbols from the executable
     instead of a DLL. Exporting symbols from an executable is a seldom
     used functionality, especially on Windows, however, it is well
     supported on both Windows and most Unix platforms. Note also that
     this approach won't work if we also use one of the databases in the
     static mode.</p>

  <p>On Windows all we have to do is place <code>person-odb.cxx</code>
     into the executable and compile it as we would in a DLL (that is,
     with the <code>PERSON_BUILD_DLL</code> macro defined). If Windows
     linker detects that an executable exports any symbols, then it
     will automatically create the corresponding import library
     (<code>person.lib</code> in our case). We then use this import
     library to build <code>person-sqlite.dll</code> and
     <code>person-pgsql.dll</code> as before.</p>

  <p>To export symbols from an executable on GNU/Linux all we need to
     do is add the <code>-rdynamic</code> option when linking our
     executable.</p>

  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="17">17 MySQL Database</a></h1>

  <p>To generate support code for the MySQL database you will need
     to pass the "<code>--database&nbsp;mysql</code>"
     (or "<code>-d&nbsp;mysql</code>") option to the ODB compiler.
     Your application will also need to link to the MySQL ODB runtime
     library (<code>libodb-mysql</code>). All MySQL-specific ODB
     classes are defined in the <code>odb::mysql</code> namespace.</p>

  <h2><a name="17.1">17.1 MySQL Type Mapping</a></h2>

  <p>The following table summarizes the default mapping between basic
     C++ value types and MySQL database types. This mapping can be
     customized on the per-type and per-member basis using the ODB
     Pragma Language (<a href="#14">Chapter 14, "ODB Pragma
     Language"</a>).</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>C++ Type</th>
      <th>MySQL Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>bool</code></td>
      <td><code>TINYINT(1)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>char</code></td>
      <td><code>CHAR(1)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>signed char</code></td>
      <td><code>TINYINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned char</code></td>
      <td><code>TINYINT UNSIGNED</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>short</code></td>
      <td><code>SMALLINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned short</code></td>
      <td><code>SMALLINT UNSIGNED</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>int</code></td>
      <td><code>INT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned int</code></td>
      <td><code>INT UNSIGNED</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>long</code></td>
      <td><code>BIGINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned long</code></td>
      <td><code>BIGINT UNSIGNED</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>long long</code></td>
      <td><code>BIGINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned long long</code></td>
      <td><code>BIGINT UNSIGNED</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>float</code></td>
      <td><code>FLOAT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>double</code></td>
      <td><code>DOUBLE</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>std::string</code></td>
      <td><code>TEXT/VARCHAR(128)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>char[N]</code></td>
      <td><code>VARCHAR(N-1)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>
  </table>

  <p>It is possible to map the <code>char</code> C++ type to an integer
     database type (for example, <code>TINYINT</code>) using the
     <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section 14.4.3,
     "<code>type</code>"</a>).</p>

  <p>Note that the <code>std::string</code> type is mapped
     differently depending on whether a member of this type
     is an object id or not. If the member is an object id,
     then for this member <code>std::string</code> is mapped
     to the <code>VARCHAR(128)</code> MySQL type. Otherwise,
     it is mapped to <code>TEXT</code>.</p>

  <p>Additionally, by default, C++ enums and C++11 enum classes are
     automatically mapped to suitable MySQL types. Contiguous
     enumerations with the zero first enumerator are mapped to
     the MySQL <code>ENUM</code> type. All other enumerations
     are mapped to the MySQL types corresponding to their
     underlying integral types (see table above). In both
     cases the default <code>NULL</code> semantics is
     <code>NOT NULL</code>. For example:</p>

  <pre class="cxx">
enum color {red, green, blue};
enum class taste: unsigned char
{
  bitter = 1, // Non-zero first enumerator.
  sweet,
  sour = 4,   // Non-contiguous.
  salty
};

#pragma db object
class object
{
  ...

  color color_; // Mapped to ENUM ('red', 'green', 'blue') NOT NULL.
  taste taste_; // Mapped to TINYNT UNSIGNED NOT NULL.
};
  </pre>

  <p>The only built-in mapping provided for the MySQL <code>DECIMAL</code>
     type is to <code>std::string/char[N]</code>, for example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  #pragma db type ("DECIMAL(6,3)")
  std::string value_;
};
  </pre>

  <p>You can, however, map <code>DECIMAL</code> to a custom C++ type by
  providing a suitable <code>odb::mysql::value_traits</code>
  specialization.</p>

  <p>It is also possible to add support for additional MySQL types,
     such as geospatial types. For more information, refer to
     <a href="#14.8">Section 14.8, "Database Type Mapping
     Pragmas"</a>.</p>

  <h3><a name="17.1.1">17.1.1 String Type Mapping</a></h3>

  <p>The MySQL ODB runtime library provides support for mapping the
     <code>std::string</code>, <code>char[N]</code>, and
     <code>std::array&lt;char, N></code> types to the MySQL <code>CHAR</code>,
     <code>VARCHAR</code>, <code>TEXT</code>, <code>NCHAR</code>, and
     <code>NVARCHAR</code> types. However, these mappings are not enabled
     by default (in particular, by default, <code>std::array</code> will
     be treated as a container). To enable the alternative mappings for
     these types we need to specify the database type explicitly using
     the <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section
     14.4.3, "<code>type</code>"</a>), for example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  #pragma db type("CHAR(2)")
  char state_[2];

  #pragma db type("VARCHAR(128)")
  std::string name_;
};
  </pre>

  <p>Alternatively, this can be done on the per-type basis, for example:</p>

  <pre class="cxx">
#pragma db value(std::string) type("VARCHAR(128)")

#pragma db object
class object
{
  ...

  std::string name_; // Mapped to VARCHAR(128).
};
  </pre>

  <p>The <code>char[N]</code> and <code>std::array&lt;char, N></code> values
     may or may not be zero-terminated. When extracting such values from the
     database, ODB will append the zero terminator if there is enough
     space.</p>

  <h3><a name="17.1.2">17.1.2 Binary Type Mapping</a></h3>

  <p>The MySQL ODB runtime library provides support for mapping the
     <code>std::vector&lt;char></code>,
     <code>std::vector&lt;unsigned&nbsp;char></code>,
     <code>char[N]</code>, <code>unsigned&nbsp;char[N]</code>,
     <code>std::array&lt;char, N></code>, and
     <code>std::array&lt;unsigned char, N></code>
     types to the MySQL <code>BINARY</code>, <code>VARBINARY</code>,
     and <code>BLOB</code> types. However, these mappings are not enabled
     by default (in particular, by default, <code>std::vector</code> and
     <code>std::array</code> will be treated as containers). To enable the
     alternative mappings for these types we need to specify the database
     type explicitly using the <code>db&nbsp;type</code> pragma
     (<a href="#14.4.3">Section 14.4.3, "<code>type</code>"</a>), for
     example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  #pragma db type("BLOB")
  std::vector&lt;char> buf_;

  #pragma db type("BINARY(16)")
  unsigned char uuid_[16];
};
  </pre>

  <p>Alternatively, this can be done on the per-type basis, for example:</p>

  <pre class="cxx">
using buffer = std::vector&lt;char>;
#pragma db value(buffer) type("BLOB")

#pragma db object
class object
{
  ...

  buffer buf_; // Mapped to BLOB.
};
  </pre>

  <p>Note also that in native queries (<a href="#4">Chapter 4, "Querying
     the Database"</a>) <code>char[N]</code> and
     <code>std::array&lt;char, N></code> parameters are by default passed
     as a string rather than a binary. To pass such parameters as a binary,
     we need to specify the database type explicitly in the
     <code>_val()</code>/<code>_ref()</code> calls. Note also that we
     don't need to do this for the integrated queries, for example:</p>

  <pre class="cxx">
char u[16] = {...};

db.query&lt;object> ("uuid = " + query::_val&lt;odb::mysql::id_blob> (u));
db.query&lt;object> (query::uuid == query::_ref (u));
  </pre>


  <h3><a name="17.1.3">17.1.3 Mixed Automatic/0 Object Id Assignment</a></h3>

  <p>In MySQL an automatic object id can also be set manually to
     <code>0</code>. For example:</p>

  <pre class="cxx">
  #pragma db id auto
  odb::nullable&lt;int64_t> id;
  </pre>

  <p>Then, when used with the <code>NO_AUTO_VALUE_ON_ZERO</code> mode, set the
    id member to <code>NULL</code> to get auto-assignment or to <code>0</code>
    to use <code>0</code> as the id. This functionality is normally used to
    assign the special <code>0</code> id to a special object.</p>


  <h2><a name="17.2">17.2 MySQL Database Class</a></h2>

  <p>The MySQL <code>database</code> class has the following
     interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace mysql
  {
    class database: public odb::database
    {
    public:
      database (const char* user,
                const char* passwd,
                const char* db,
                const char* host = 0,
                unsigned int port = 0,
                const char* socket = 0,
                const char* charset = 0,
                unsigned long client_flags = 0,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      database (const std::string&amp; user,
                const std::string&amp; passwd,
                const std::string&amp; db,
                const std::string&amp; host = "",
                unsigned int port = 0,
                const std::string* socket = 0,
                const std::string&amp; charset = "",
                unsigned long client_flags = 0,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      database (const std::string&amp; user,
                const std::string* passwd,
                const std::string&amp; db,
                const std::string&amp; host = "",
                unsigned int port = 0,
                const std::string* socket = 0,
                const std::string&amp; charset = "",
                unsigned long client_flags = 0,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      database (const std::string&amp; user,
                const std::string&amp; passwd,
                const std::string&amp; db,
                const std::string&amp; host,
                unsigned int port,
                const std::string&amp; socket,
                const std::string&amp; charset = "",
                unsigned long client_flags = 0,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      database (const std::string&amp; user,
                const std::string* passwd,
                const std::string&amp; db,
                const std::string&amp; host,
                unsigned int port,
                const std::string&amp; socket,
                const std::string&amp; charset = "",
                unsigned long client_flags = 0,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      database (int&amp; argc,
                char* argv[],
                bool erase = false,
                const std::string&amp; charset = "",
                unsigned long client_flags = 0,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      static void
      print_usage (std::ostream&amp;);

    public:
      const char*
      user () const;

      const char*
      password () const;

      const char*
      db () const;

      const char*
      host () const;

      unsigned int
      port () const;

      const char*
      socket () const;

      const char*
      charset () const;

      unsigned long
      client_flags () const;

    public:
      connection_ptr
      connection ();
    };
  }
}
  </pre>

  <p>You will need to include the <code>&lt;odb/mysql/database.hxx></code>
     header file to make this class available in your application.</p>

  <p>The overloaded <code>database</code> constructors allow us
     to specify MySQL database parameters that should be used when
     connecting to the database. In MySQL <code>NULL</code> and an
     empty string are treated as the same values for all the
     string parameters except <code>password</code> and
     <code>socket</code>.</p>

  <p>The <code>charset</code> argument allows us to specify the client
     character set, that is, the character set in which the application
     will encode its text data. Note that this can be different from
     the MySQL server character set. If this argument is not specified or
     is empty, then the default MySQL client character set is used, normally
     <code>latin1</code>. Commonly used values for this argument are
     <code>latin1</code> (equivalent to Windows cp1252 and similar to
     ISO-8859-1) and <code>utf8</code>. For other possible values
     as well as more information on character set support in MySQL,
     refer to the MySQL documentation.</p>

  <p>The <code>client_flags</code> argument allows us to specify various
     MySQL client library flags. For more information on the possible
     values, refer to the MySQL C API documentation. The
     <code>CLIENT_FOUND_ROWS</code> flag is always set by the MySQL ODB
     runtime regardless of whether it was passed in the
     <code>client_flags</code> argument.</p>

  <p>The last constructor extracts the database parameters
     from the command line. The following options are recognized:</p>

  <pre class="terminal">
  --user &lt;login>
  --password &lt;password>
  --database &lt;name>
  --host &lt;host>
  --port &lt;integer>
  --socket &lt;socket>
  --options-file &lt;file>
  </pre>

  <p>The <code>--options-file</code> option allows us to specify some
     or all of the database options in a file with each option appearing
     on a separate line followed by a space and an option value.</p>

  <p>If the <code>erase</code> argument to this constructor is true,
     then the above options are removed from the <code>argv</code>
     array and the <code>argc</code> count is updated accordingly.
     This is primarily useful if your application accepts other
     options or arguments and you would like to get the MySQL
     options out of the <code>argv</code> array.</p>

  <p>This constructor throws the <code>odb::mysql::cli_exception</code>
     exception if the MySQL option values are missing or invalid.
     See section <a href="#17.4">Section 17.4, "MySQL Exceptions"</a>
     for more information on this exception.</p>

  <p>The static <code>print_usage()</code> function prints the list of options
     with short descriptions that are recognized by this constructor.</p>

  <p>The last argument to all of the constructors is a pointer to the
     connection factory. In C++98/03, it is <code>std::auto_ptr</code> while
     in C++11 <code>std::unique_ptr</code> is used instead. If we pass a
     non-<code>NULL</code> value, the database instance assumes ownership
     of the factory instance. The connection factory interface as well as
     the available implementations are described in the next section.</p>

  <p>The set of accessor functions following the constructors allows us
     to query the parameters of the <code>database</code> instance.</p>

  <p>The <code>connection()</code> function returns a pointer to the
     MySQL database connection encapsulated by the
     <code>odb::mysql::connection</code> class. For more information
     on <code>mysql::connection</code>, refer to <a href="#17.3">Section
     17.3, "MySQL Connection and Connection Factory"</a>.</p>

  <h2><a name="17.3">17.3 MySQL Connection and Connection Factory</a></h2>

  <p>The <code>mysql::connection</code> class has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace mysql
  {
    class connection: public odb::connection
    {
    public:
      connection (database&amp;);
      connection (database&amp;, MYSQL*);

      MYSQL*
      handle ();
    };

    using connection_ptr = details::shared_ptr&lt;connection>;
  }
}
  </pre>

  <p>For more information on the <code>odb::connection</code> interface,
     refer to <a href="#3.6">Section 3.6, "Connections"</a>. The first
     overloaded <code>mysql::connection</code> constructor establishes a
     new MySQL connection. The second constructor allows us to create
     a <code>connection</code> instance by providing an already connected
     native MySQL handle. Note that the <code>connection</code>
     instance assumes ownership of this handle. The <code>handle()</code>
     accessor returns the MySQL handle corresponding to the connection.</p>

  <p>The <code>mysql::connection_factory</code> abstract class has the
     following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace mysql
  {
    class connection_factory
    {
    public:
      virtual void
      database (database&amp;) = 0;

      virtual connection_ptr
      connect () = 0;
    };
  }
}
  </pre>

  <p>The <code>database()</code> function is called when a connection
     factory is associated with a database instance. This happens in
     the <code>odb::mysql::database</code> class constructors. The
     <code>connect()</code> function is called whenever a database
     connection is requested.</p>

  <p>The two implementations of the <code>connection_factory</code>
     interface provided by the MySQL ODB runtime are
     <code>new_connection_factory</code> and
     <code>connection_pool_factory</code>. You will need to include
     the <code>&lt;odb/mysql/connection-factory.hxx></code>
     header file to make the <code>connection_factory</code> interface
     and these implementation classes available in your application.</p>

  <p>The <code>new_connection_factory</code> class creates a new
     connection whenever one is requested. When a connection is no
     longer needed, it is released and closed. The
     <code>new_connection_factory</code> class has the following
     interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace mysql
  {
    class new_connection_factory: public connection_factory
    {
    public:
      new_connection_factory ();
    };
};
  </pre>

  <p>The <code>connection_pool_factory</code> class implements a
     connection pool. It has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace mysql
  {
    class connection_pool_factory: public connection_factory
    {
    public:
      connection_pool_factory (std::size_t max_connections = 0,
                               std::size_t min_connections = 0,
                               bool ping = true);

    protected:
      class pooled_connection: public connection
      {
      public:
        pooled_connection (database_type&amp;);
        pooled_connection (database_type&amp;, MYSQL*);
      };

      using pooled_connection_ptr = details::shared_ptr&lt;pooled_connection>;

      virtual pooled_connection_ptr
      create ();
    };
};
  </pre>

  <p>The <code>max_connections</code> argument in the
     <code>connection_pool_factory</code> constructor specifies the maximum
     number of concurrent connections that this pool factory will
     maintain. Similarly, the <code>min_connections</code> argument
     specifies the minimum number of available connections that
     should be kept open. The <code>ping</code> argument specifies
     whether the factory should validate the connection before
     returning it to the caller.</p>

  <p>Whenever a connection is requested, the pool factory first
     checks if there is an unused connection that can be returned.
     If there is none, the pool factory checks the
     <code>max_connections</code> value to see if a new connection
     can be created. If the total number of connections maintained
     by the pool is less than this value, then a new connection is
     created and returned. Otherwise, the caller is blocked until
     a connection becomes available.</p>

  <p>When a connection is released, the pool factory first checks
     if there are blocked callers waiting for a connection. If so, then
     one of them is unblocked and is given the connection. Otherwise,
     the pool factory checks whether the total number of connections
     maintained by the pool is greater than the <code>min_connections</code>
     value. If that's the case, the connection is closed. Otherwise, the
     connection is added to the pool of available connections to be
     returned on the next request. In other words, if the number of
     connections maintained by the pool exceeds <code>min_connections</code>
     and there are no callers waiting for a new connection,
     then the pool will close the excess connections.</p>

  <p>If the <code>max_connections</code> value is 0, then the pool will
     create a new connection whenever all of the existing connections
     are in use. If the <code>min_connections</code> value is 0, then
     the pool will never close a connection and instead maintain all
     the connections that were ever created.</p>

  <p>Connection validation (the <code>ping</code> argument) is useful
     if your application may experience long periods of inactivity. In
     such cases the MySQL server may close network connections that have
     been inactive for too long. If during connection validation the pool
     factory detects that the connection has been terminated, it silently
     closes it and tries to find or create another connection instead.</p>

  <p>The <code>create()</code> virtual function is called whenever the
     pool needs to create a new connection. By deriving from the
     <code>connection_pool_factory</code> class and overriding this
     function we can implement custom connection establishment
     and configuration.</p>

  <p>If you pass <code>NULL</code> as the connection factory to
     one of the <code>database</code> constructors, then the
     <code>connection_pool_factory</code> instance will be
     created by default with the min and max connections values
     set to <code>0</code> and connection validation enabled.
     The following code fragment shows how we can pass our own
     connection factory instance:</p>

  <pre class="cxx">
#include &lt;odb/database.hxx>

#include &lt;odb/mysql/database.hxx>
#include &lt;odb/mysql/connection-factory.hxx>

int
main (int argc, char* argv[])
{
  unique_ptr&lt;odb::mysql::connection_factory> f (
    new odb::mysql::connection_pool_factory (20));

  unique_ptr&lt;odb::database> db (
    new mysql::database (argc, argv, false, 0, f));
}
  </pre>

  <h2><a name="17.4">17.4 MySQL Exceptions</a></h2>

  <p>The MySQL ODB runtime library defines the following MySQL-specific
     exceptions:</p>

  <pre class="cxx">
namespace odb
{
  namespace mysql
  {
    class database_exception: odb::database_exception
    {
    public:
      unsigned int
      error () const;

      const std::string&amp;
      sqlstate () const;

      const std::string&amp;
      message () const;

      virtual const char*
      what () const throw ();
    };

    class cli_exception: odb::exception
    {
    public:
      virtual const char*
      what () const throw ();
    };
  }
}
  </pre>

  <p>You will need to include the <code>&lt;odb/mysql/exceptions.hxx></code>
     header file to make these exceptions available in your application.</p>

  <p>The <code>odb::mysql::database_exception</code> is thrown if
     a MySQL database operation fails. The MySQL-specific error
     information is accessible via the <code>error()</code>,
     <code>sqlstate()</code>, and <code>message()</code> functions.
     All this information is also combined and returned in a
     human-readable form by the <code>what()</code> function.</p>

  <p>The <code>odb::mysql::cli_exception</code> is thrown by the
     command line parsing constructor of the <code>odb::mysql::database</code>
     class if the MySQL option values are missing or invalid. The
     <code>what()</code> function returns a human-readable description
     of an error.</p>

  <h2><a name="17.5">17.5 MySQL Limitations</a></h2>

  <p>The following sections describe MySQL-specific limitations imposed
     by the current MySQL and ODB runtime versions.</p>

  <h3><a name="17.5.1">17.5.1 Foreign Key Constraints</a></h3>

  <p>ODB relies on standard SQL behavior which requires that foreign
     key constraints checking is deferred until the transaction is
     committed. The only behaviors supported by MySQL are to either
     check such constraints immediately (InnoDB engine) or to ignore
     foreign key constraints altogether (all other engines). As a
     result, by default, schemas generated by the ODB compiler for
     MySQL have foreign key definitions commented out. They are
     retained only for documentation.</p>

  <p>You can override the default behavior and instruct the ODB
     compiler to generate non-deferrable foreign keys by specifying
     the <code>--fkeys-deferrable-mode not_deferrable</code> ODB
     compiler option. Note, however, that in this case the order in
     which you persist, update, and erase objects within a transaction
     becomes important.</p>

  <h2><a name="17.6">17.6 MySQL Index Definitions</a></h2>

  <p>When the <code>index</code> pragma (<a href="#14.7">Section 14.7,
     "Index Definition Pragmas"</a>) is used to define a MySQL index,
     the <code>type</code> clause specifies the index type (for example,
     <code>UNIQUE</code>, <code>FULLTEXT</code>, <code>SPATIAL</code>),
     the <code>method</code> clause specifies the index method (for
     example, <code>BTREE</code>, <code>HASH</code>), and the
     <code>options</code> clause is not used. The column options
     can be used to specify column length limits and the sort order.
     For example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  std::string name_;

  #pragma db index method("HASH") member(name_, "(100) DESC")
};
  </pre>

  <h2><a name="17.7">17.7 MySQL Stored Procedures</a></h2>

  <p>ODB native views (<a href="#10.6">Section 10.6, "Native Views"</a>)
     can be used to call MySQL stored procedures. For example, assuming
     we are using the <code>person</code> class from <a href="#2">Chapter
     2, "Hello World Example"</a> (and the corresponding <code>person</code>
     table), we can create a stored procedure that given the min and max
     ages returns some information about all the people in that range:</p>

  <pre class="sql">
CREATE PROCEDURE person_range (
  IN min_age SMALLINT,
  IN max_age SMALLINT)
BEGIN
  SELECT age, first, last FROM person
    WHERE age >= min_age AND age &lt;= max_age;
END
  </pre>

  <p>Given the above stored procedure we can then define an ODB view
     that can be used to call it and retrieve its result:</p>

  <pre class="cxx">
#pragma db view query("CALL person_range((?))")
struct person_range
{
  unsigned short age;
  std::string first;
  std::string last;
};
  </pre>

  <p>The following example shows how we can use the above view to
     print the list of people in a specific age range:</p>

  <pre class="cxx">
using query = odb::query&lt;person_range>;
using result = odb::result&lt;person_range>;

transaction t (db.begin ());

result r (
  db.query&lt;person_range> (
    query::_val (1) + "," + query::_val (18)));

for (result::iterator i (r.begin ()); i != r.end (); ++i)
  cerr &lt;&lt; i->first &lt;&lt; " " &lt;&lt; i->last &lt;&lt; " " &lt;&lt; i->age &lt;&lt; endl;

t.commit ();
  </pre>

   <p>Note that as with all native views, the order and types of data members
      must match those of columns in the <code>SELECT</code> list inside
      the stored procedure.</p>

   <p>There are also a number of limitations when it comes to support for
      MySQL stored procedures in ODB views. First of all, you have to use
      MySQL server and client libraries version 5.5.3 or later since this
      is the version in which support for calling stored procedures with
      prepared statements was first added (the
      <code>mysql_stmt_next_result()</code> function).</p>

   <p>In MySQL, a stored procedure can produce multiple results.
      For example, if a stored procedure executes several
      <code>SELECT</code> statements, then the result of calling such a
      procedure consists of multiple row sets, one for
      each <code>SELECT</code> statement. Additionally, if the procedure has
      any <code>OUT</code> or <code>INOUT</code> parameters, then their values
      are returned as an additional special row set containing only a single
      row.  Because such multiple row sets can contain varying number and type
      of columns, they cannot be all extracted into a single view. As a
      result, an ODB view will only extract the data from the first row set
      and ignore all the subsequent ones.</p>

   <p>In particular, this means that we can use an ODB view to extract
      the values of the <code>OUT</code> and <code>INOUT</code>
      parameters provided that the stored procedure does not generate
      any other row sets. For example:</p>

  <pre class="sql">
CREATE PROCEDURE person_min_max_age (
  OUT min_age SMALLINT,
  OUT max_age SMALLINT)
BEGIN
  SELECT MIN(age), MAX(age) INTO min_age, max_age FROM person;
END
  </pre>

  <pre class="cxx">
#pragma db view query("CALL person_min_max_age((?))")
struct person_min_max_age
{
  unsigned short min_age;
  unsigned short max_age;
};
  </pre>

  <pre class="cxx">
using query = odb::query&lt;person_min_max_age>;

transaction t (db.begin ());

// We know this query always returns a single row, so use query_value().
// We have to pass dummy values for OUT parameters.
//
person_min_max_age mma (
  db.query_value&lt;person_min_max_age> (
    query::_val (0) + "," + query::_val (0)));

cerr &lt;&lt; mma.min_age &lt;&lt; " " &lt;&lt; mma.max_age &lt;&lt; endl;

t.commit ();
  </pre>

  <p>Another limitation that stems from having multiple results is the
     inability to cache the result of a stored procedure call. In
     other words, a MySQL stored procedure call always produces an
     uncached query result (<a href="#4.4">Section 4.4, "Query
     Result"</a>).</p>

  <hr class="page-break"/>
  <h1><a name="18">18 SQLite Database</a></h1>

  <p>To generate support code for the SQLite database you will need
     to pass the "<code>--database&nbsp;sqlite</code>"
     (or "<code>-d&nbsp;sqlite</code>") option to the ODB compiler.
     Your application will also need to link to the SQLite ODB runtime
     library (<code>libodb-sqlite</code>). All SQLite-specific ODB
     classes are defined in the <code>odb::sqlite</code> namespace.</p>

  <h2><a name="18.1">18.1 SQLite Type Mapping</a></h2>

  <p>The following table summarizes the default mapping between basic
     C++ value types and SQLite database types. This mapping can be
     customized on the per-type and per-member basis using the ODB
     Pragma Language (<a href="#14">Chapter 14, "ODB Pragma
     Language"</a>).</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>C++ Type</th>
      <th>SQLite Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>bool</code></td>
      <td><code>INTEGER</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>char</code></td>
      <td><code>TEXT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>signed char</code></td>
      <td><code>INTEGER</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned char</code></td>
      <td><code>INTEGER</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>short</code></td>
      <td><code>INTEGER</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned short</code></td>
      <td><code>INTEGER</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>int</code></td>
      <td><code>INTEGER</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned int</code></td>
      <td><code>INTEGER</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>long</code></td>
      <td><code>INTEGER</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned long</code></td>
      <td><code>INTEGER</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>long long</code></td>
      <td><code>INTEGER</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned long long</code></td>
      <td><code>INTEGER</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>float</code></td>
      <td><code>REAL</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>double</code></td>
      <td><code>REAL</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>std::string</code></td>
      <td><code>TEXT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>char[N]</code></td>
      <td><code>TEXT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>std::wstring (Windows only)</code></td>
      <td><code>TEXT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>wchar_t[N] (Windows only)</code></td>
      <td><code>TEXT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>odb::sqlite::text</code></td>
      <td><code>TEXT (STREAM)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>odb::sqlite::blob</code></td>
      <td><code>BLOB (STREAM)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>
  </table>

  <p>It is possible to map the <code>char</code> C++ type to the
     <code>INTEGER</code> SQLite type using the <code>db&nbsp;type</code>
     pragma (<a href="#14.4.3">Section 14.4.3, "<code>type</code>"</a>).</p>

  <p>SQLite represents the <code>NaN</code> <code>FLOAT</code> value
     as a <code>NULL</code> value. As a result, columns of the
     <code>float</code> and <code>double</code> types are by default
     declared as <code>NULL</code>. However, you can override this by
     explicitly declaring them as <code>NOT NULL</code> with the
     <code>db&nbsp;not_null</code> pragma (<a href="#14.4.6">Section
     14.4.6, "<code>null/not_null</code>"</a>).</p>

  <p>Additionally, by default, C++ enums and C++11 enum classes are
     automatically mapped to the SQLite <code>INTEGER</code> type with
     the default <code>NULL</code> semantics being <code>NOT NULL</code>.
     For example:</p>

  <pre class="cxx">
enum color {red, green, blue};
enum class taste: unsigned char
{
  bitter = 1,
  sweet,
  sour = 4,
  salty
};

#pragma db object
class object
{
  ...

  color color_; // Automatically mapped to INTEGER.
  taste taste_; // Automatically mapped to INTEGER.
};
  </pre>

  <p>Note also that SQLite only operates with signed integers and the largest
     value that an SQLite database can store is a signed 64-bit integer. As
     a result, greater <code>unsigned&nbsp;long</code> and
     <code>unsigned&nbsp;long&nbsp;long</code> values will be represented in
     the database as negative values.</p>

  <p>It is also possible to add support for additional SQLite types,
     such as <code>NUMERIC</code>. For more information, refer to
     <a href="#14.8">Section 14.8, "Database Type Mapping
     Pragmas"</a>.</p>

  <h3><a name="18.1.1">18.1.1 String Type Mapping</a></h3>

  <p>The SQLite ODB runtime library provides support for mapping the
     <code>std::array&lt;char, N></code> and, on Windows,
     <code>std::array&lt;wchar_t, N></code> types to the SQLite
     <code>TEXT</code> type. However, this mapping is not enabled by
     default (in particular, by default, <code>std::array</code> will
     be treated as a container). To enable the alternative mapping for
     this type we need to specify the database type explicitly using
     the <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section
     14.4.3, "<code>type</code>"</a>), for example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  #pragma db type("TEXT")
  std::array&lt;char, 128> name_;
};
  </pre>

  <p>Alternatively, this can be done on the per-type basis, for example:</p>

  <pre class="cxx">
using name_type = std::array&lt;char, 128>;
#pragma db value(name_type) type("TEXT")

#pragma db object
class object
{
  ...

  name_type name_; // Mapped to TEXT.
};
  </pre>

  <p>The <code>char[N]</code>, <code>std::array&lt;char, N></code>,
     <code>wchar_t[N]</code>, and <code>std::array&lt;wchar_t, N></code>
     values may or may not be zero-terminated. When extracting such values
     from the database, ODB will append the zero terminator if there is
     enough space.</p>

  <h3><a name="18.1.2">18.1.2 Binary Type Mapping</a></h3>

  <p>The SQLite ODB runtime library provides support for mapping the
     <code>std::vector&lt;char></code>,
     <code>std::vector&lt;unsigned&nbsp;char></code>,
     <code>char[N]</code>, <code>unsigned&nbsp;char[N]</code>,
     <code>std::array&lt;char, N></code>, and
     <code>std::array&lt;unsigned char, N></code>
     types to the SQLite <code>BLOB</code> type. However, these mappings
     are not enabled by default (in particular, by default,
     <code>std::vector</code> and <code>std::array</code> will be treated
     as containers). To enable the alternative mappings for these types
     we need to specify the database type explicitly using the
     <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section 14.4.3,
     "<code>type</code>"</a>), for example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  #pragma db type("BLOB")
  std::vector&lt;char> buf_;

  #pragma db type("BLOB")
  unsigned char uuid_[16];
};
  </pre>

  <p>Alternatively, this can be done on the per-type basis, for example:</p>

  <pre class="cxx">
using buffer = std::vector&lt;char>;
#pragma db value(buffer) type("BLOB")

#pragma db object
class object
{
  ...

  buffer buf_; // Mapped to BLOB.
};
  </pre>

  <p>Note also that in native queries (<a href="#4">Chapter 4, "Querying
     the Database"</a>) <code>char[N]</code> and
     <code>std::array&lt;char, N></code> parameters are by default passed
     as a string rather than a binary. To pass such parameters as a binary,
     we need to specify the database type explicitly in the
     <code>_val()</code>/<code>_ref()</code> calls. Note also that we
     don't need to do this for the integrated queries, for example:</p>

  <pre class="cxx">
char u[16] = {...};

db.query&lt;object> ("uuid = " + query::_val&lt;odb::sqlite::id_blob> (u));
db.query&lt;object> (query::uuid == query::_ref (u));
  </pre>

  <h3><a name="18.1.3">18.1.3 Incremental <code>BLOB</code>/<code>TEXT</code> I/O</a></h3>

  <p>This section describes the SQLite ODB runtime library support for
     incremental reading and writing of <code>BLOB</code> and
     <code>TEXT</code> values. The provided API is a thin wrapper
     around the native SQLite <code>sqlite3_blob_*()</code> function
     family. As a result, it is highly recommended that you familiarize
     yourself with the semantics of this SQLite functionality before
     continuing with this section.</p>

  <p>The SQLite runtime provides the <code>blob</code> and
     <code>text</code> types that can be used to represent
     <code>BLOB</code> and <code>TEXT</code> data members
     that will be read/written using the incremental I/O.
     For example:</p>

  <pre class="cxx">
#include &lt;odb/sqlite/blob.hxx>
#include &lt;odb/sqlite/text.hxx>

#pragma db object
class object
{
public
  #pragma db id auto
  unsigned long long id;

  odb::sqlite::blob b; // Mapped to BLOB.
  odb::sqlite::text t; // Mapped to TEXT.
};
  </pre>

  <p>The <code>blob</code> and <code>text</code> types should be
     viewed as <em>descriptors</em> of the <code>BLOB</code> and
     <code>TEXT</code> values (rather than the values themselves)
     that can be used to <em>open</em> the values for reading or
     writing. These two types have an identical interface that
     is presented below. Notice that it is essentially the list
     of arguments (except for <code>size</code> which is discussed
     below) to the <code>sqlite3_blob_open()</code> function:</p>

  <pre class="cxx">
namespace odb
{
  namespace sqlite
  {
    class blob|text
    {
    public:
      explicit
      blob|text (std::size_t = 0);

      std::size_t size ()
      void        size (std::size_t);

      const std::string&amp; db () const;
      const std::string&amp; table () const;
      const std::string&amp; column () const;
      long long          rowid () const;

      void
      clear ();
    };
  }
}
  </pre>

  <p>To read/write data from/to a incremental <code>BLOB</code> or
     <code>TEXT</code> value we use the corresponding
     <code>blob_stream</code> and <code>text_stream</code>
     stream types. Their interfaces closely mimic the
     underlying <code>sqlite3_blob_*()</code> functions
     and are presented below. Note that in order to create
     a stream we have to pass the corresponding descriptor:</p>

  <pre class="cxx">
#include &lt;odb/sqlite/stream.hxx>

namespace odb
{
  namespace sqlite
  {
    class stream
    {
    public:
      stream (const char* db,
              const char* table,
              const char* column,
              long long rowid,
              bool rw);

      std::size_t
      size () const;

      // The following two functions throw std::invalid_argument if
      // offset + n is past size().
      //
      void
      read (void* buf, std::size_t n, std::size_t offset = 0);

      void
      write (const void* buf, std::size_t n, std::size_t offset = 0);

      sqlite3_blob*
      handle () const;

      // Close without reporting errors, if any.
      //
      ~stream ();

      // Close with reporting errors, if any.
      //
      void
      close ();

      // Open the same BLOB but in a different row. Can be faster
      // than creating a new stream instance. Note that the stream
      // must be in the open state prior to calling this function.
      //
      void
      reopen (long long rowid);
    };
  }
}

#include &lt;odb/sqlite/blob-stream.hxx>

namespace odb
{
  namespace sqlite
  {
    class blob_stream: public stream
    {
    public:
      blob_stream (const blob&amp;, bool rw);
    };
  }
}

#include &lt;odb/sqlite/text-stream.hxx>

namespace odb
{
  namespace sqlite
  {
    class text_stream: public stream
    {
    public:
      text_stream (const text&amp;, bool rw);
    };
  }
}
  </pre>

  <p>The <code>rw</code> argument to the constructors above
     specifies whether to open the value for reading only
     (<code>false</code>) or to read and write
     (<code>true</code>).</p>

  <p>In SQLite the incremental <code>BLOB</code> and
     <code>TEXT</code> sizes are fixed in the sense that
     they must be specified before the object is persisted
     or updated and the following write operations can
     only write that much data. This is what the <code>size</code>
     data member in the descriptors is for. You can also determine
     the size of the opened value, for both reading and writing,
     using the <code>size()</code> stream function. The
     following example puts all of this together:</p>

  <pre class="cxx">
#include &lt;odb/sqlite/blob-stream.hxx>
#include &lt;odb/sqlite/text-stream.hxx>

string txt (1024 * 1024, 't');
vector&lt;char> blb (1024 * 1024, 'b');

object o;

// Persist.
//
{
  transaction tx (db.begin ());

  // Specify the sizes of the values before calling persist().
  //
  o.t.size (txt.size ());
  o.b.size (blb.size ());

  db.persist (o);

  // Write the data.
  //
  blob_stream bs (o.b, true); // Open for read/write.
  assert (bs.size () == blb.size ());
  bs.write (blb.data (), blb.size ());

  text_stream ts (o.t, true); // Open for read/write.
  assert (ts.size () == txt.size ());
  ts.write (txt.data (), txt.size ());

  tx.commit ();
}

// Load.
//
{
  transaction tx (db.begin ());
  unique_ptr&lt;object> p (db.load&lt;object> (o.id));

  text_stream ts (p->t, false); // Open for reading.
  vector&lt;char> t (ts.size () + 1, '\0');
  ts.read (t.data (), t.size () - 1);
  assert (string (t.data ()) == txt);

  blob_stream bs (p->b, false); // Open for reading.
  vector&lt;char> b (bs.size (), '\0');
  bs.read (b.data (), b.size ());
  assert (b == blb);

  tx.commit ();
}

// Update
//
txt.resize (txt.size () + 1, 't');
txt[0] = 'A';
txt[txt.size () - 1] = 'Z';

blb.resize (blb.size () - 1);
blb.front () = 'A';
blb.back () = 'Z';

{
  transaction tx (db.begin ());

  // Specify the new sizes of the values before calling update().
  //
  o.t.size (txt.size ());
  o.b.size (blb.size ());

  db.update (o);

  // Write the data.
  //
  blob_stream bs (o.b, true);
  bs.write (blb.data (), blb.size ());

  text_stream ts (o.t, true);
  ts.write (txt.data (), txt.size ());

  tx.commit ();
}
  </pre>

  <p>For the most part, the incremental <code>BLOB</code> and
     <code>TEXT</code> descriptors can be used as any other
     simple values. Specifically, they can be used as container
     elements (<a href="#5">Chapter 5, "Containers"</a>), as
     <code>NULL</code>-able values (<a href="#7.3">Section 7.3,
     "Pointers and NULL Value Semantics"</a>), and in views
     (<a href="#10">Chapter 10, "Views"</a>). The following
     example illustrates the use within a view:</p>

  <pre class="cxx">
#pragma db view object(object)
struct load_b
{
  odb::sqlite::blob b;
};

using query = odb::query&lt;load_b>;

transaction tx (db.begin ());

for (load_b&amp; lb: db.query&lt;load_b> (query::t == "test"))
{
  blob_stream bs (lb.b, false);
  vector&lt;char> b (bs.size (), '\0');
  bs.read (b.data (), b.size ());
}

tx.commit ();
  </pre>

  <p>However, being a low-level, SQLite-specific mechanism, the
     incremental I/O has a number of nuances that should be kept in
     mind. Firstly, the streams should be opened within a transaction
     and, unless already closed, they will be automatically closed
     when the transaction is committed or rolled back. The following
     modification of the previous example helps to illustrate this
     point:</p>

  <pre class="cxx">
{
  transaction tx (db.begin ());

  // ...

  db.persist (o);

  blob_stream bs (o.b, true);

  tx.commit ();

  // ERROR: stream is closed.
  //
  bs.write (blb.data (), blb.size ());
}

// ERROR: not in transaction.
//
text_stream ts (o.t, true);
  </pre>

  <p>Because loading an object with an incremental <code>BLOB</code> or
     <code>TEXT</code> value involves additional actions after the
     database function returns (that is, reading the actual data),
     certain commonly-expected "round-trip" assumptions will no
     longer hold unless special steps are taken, for instance
     (again, continuing with our example):</p>

  <pre class="cxx">
transaction tx (db.begin ());

unique_ptr&lt;object> p (db.load&lt;object> (o.id));
p->name = "foo"; // Update some other member.
db.update (*p);  // Bad behavior: incremental BLOB/TEXT invalidated.

tx.commit ();
  </pre>

  <p>One way to restore the expected behavior is to place the
     incremental <code>BLOB</code> and <code>TEXT</code> values
     into their own, separately loaded/updated sections
     (<a href="#9">Chapter 9, "Sections"</a>). The alternative
     approach would be to perform the incremental I/O as part
     of the database operation <code>post_*</code> callbacks
     (<a href="#14.1.7">Section 14.1.7, "<code>callback</code>"</a>).</p>

  <p>Finally, note that when using incremental <code>TEXT</code>
     values, the data that we read/write is the raw bytes in
     the encoding used by the database (<code>UTF-8</code> by
     default; see SQLite <code>PRAGMA encoding</code> documentation
     for details).</p>

  <h3><a name="18.1.4">18.1.4 Mixed Automatic/Manual Object Id Assignment</a></h3>

  <p>In SQLite an automatic object id can also be assigned manually. For
     example:</p>

  <pre class="cxx">
  #pragma db id auto
  odb::nullable&lt;int64_t> id;
  </pre>

  <p>Then set the id member to <code>NULL</code> to get auto-assignment or to
     the actual value to use a manual id. This functionality is normally
     used to reserve a special id, typically <code>0</code>, for a special
     object.</p>


  <h2><a name="18.2">18.2 SQLite Database Class</a></h2>

  <p>The SQLite <code>database</code> class has the following
     interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace sqlite
  {
    class database: public odb::database
    {
    public:
      database (const std::string&amp; name,
                int flags = SQLITE_OPEN_READWRITE,
                bool foreign_keys = true,
                const std::string&amp; vfs = "",
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

#ifdef _WIN32
      database (const std::wstring&amp; name,
                int flags = SQLITE_OPEN_READWRITE,
                bool foreign_keys = true,
                const std::string&amp; vfs = "",
                std::[auto|unique]_ptr&lt;connection_factory> = 0);
#endif

      database (int&amp; argc,
                char* argv[],
                bool erase = false,
                int flags = SQLITE_OPEN_READWRITE,
                bool foreign_keys = true,
                const std::string&amp; vfs = "",
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      static void
      print_usage (std::ostream&amp;);

    public:
      const std::string&amp;
      name () const;

      int
      flags () const;

    public:
      transaction
      begin_immediate ();

      transaction
      begin_exclusive ();

    public:
      connection_ptr
      connection ();
    };
  }
}
  </pre>

  <p>You will need to include the <code>&lt;odb/sqlite/database.hxx></code>
     header file to make this class available in your application.</p>

  <p>The first constructor opens the specified SQLite database. The
     <code>name</code> argument is the database file name to open in
     the UTF-8 encoding. If this argument is empty, then a temporary,
     on-disk database is created. If this argument is the
     <code>:memory:</code> special value, then a temporary, in-memory
     database is created. The <code>flags</code> argument allows us to
     specify SQLite opening flags. For more information on the possible
     values, refer to the <code>sqlite3_open_v2()</code> function description
     in the SQLite C API documentation. The <code>foreign_keys</code>
     argument specifies whether foreign key constraints checking
     should be enabled. See <a href="#18.6.3">Section 18.6.3,
     "Foreign Key Constraints"</a> for more information on foreign
     keys. The <code>vfs</code> argument specifies the SQLite
     virtual file system module that should be used to access the
     database. If this argument is empty, then the default vfs module
     is used. Again, refer to the <code>sqlite3_open_v2()</code> function
     documentation for detail.</p>

  <p>The following example shows how we can open the <code>test.db</code>
     database in the read-write mode and create it if it does not exist:</p>

  <pre class="cxx">
unique_ptr&lt;odb::database> db (
  new odb::sqlite::database (
    "test.db",
    SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE));
  </pre>

  <p>The second constructor is the same as the first except that the database
     name is passes as <code>std::wstring</code> in the UTF-16 encoding. This
     constructor is only available when compiling for Windows.</p>

  <p>The third constructor extracts the database parameters from the
     command line. The following options are recognized:</p>

  <pre class="terminal">
  --database &lt;name>
  --create
  --read-only
  --options-file &lt;file>
  </pre>

  <p>By default, this constructor opens the database in the read-write mode
     (<code>SQLITE_OPEN_READWRITE</code> flag). If the <code>--create</code>
     flag is specified, then the database file is created if it does
     not already exist (<code>SQLITE_OPEN_CREATE</code> flag). If the
     <code>--read-only</code> flag is specified, then the database is
     opened in the read-only mode (<code>SQLITE_OPEN_READONLY</code>
     flag instead of <code>SQLITE_OPEN_READWRITE</code>). The
     <code>--options-file</code> option allows us to specify some
     or all of the database options in a file with each option appearing
     on a separate line followed by a space and an option value.</p>

  <p>If the <code>erase</code> argument to this constructor is true,
     then the above options are removed from the <code>argv</code>
     array and the <code>argc</code> count is updated accordingly.
     This is primarily useful if your application accepts other
     options or arguments and you would like to get the SQLite
     options out of the <code>argv</code> array.</p>

  <p>The <code>flags</code> argument has the same semantics as in
     the first constructor. Flags from the command line always override
     the corresponding values specified with this argument.</p>

  <p>The third constructor throws the <code>odb::sqlite::cli_exception</code>
     exception if the SQLite option values are missing or invalid.
     See <a href="#18.5">Section 18.5, "SQLite Exceptions"</a>
     for more information on this exception.</p>

  <p>The static <code>print_usage()</code> function prints the list of options
     with short descriptions that are recognized by the third constructor.</p>

  <p>The last argument to all of the constructors is a pointer to the
     connection factory. In C++98/03, it is <code>std::auto_ptr</code> while
     in C++11 <code>std::unique_ptr</code> is used instead. If we pass a
     non-<code>NULL</code> value, the database instance assumes ownership
     of the factory instance. The connection factory interface as well as
     the available implementations are described in the next section.</p>

  <p>The set of accessor functions following the constructors allows us
     to query the parameters of the <code>database</code> instance.</p>

  <p>The <code>begin_immediate()</code> and <code>begin_exclusive()</code>
     functions are the SQLite-specific extensions to the standard
     <code>odb::database::begin()</code> function (see
     <a href="#3.5">Section 3.5, "Transactions"</a>). They allow us
     to start an immediate (<code>BEGIN IMMEDIATE</code>) and an exclusive
     (<code>BEGIN EXCLUSIVE</code>) SQLite transaction, respectively.
     For more information on the semantics of the immediate and exclusive
     transactions, refer to the <code>BEGIN</code> statement description
     in the SQLite documentation.</p>

  <p>The <code>connection()</code> function returns a pointer to the
     SQLite database connection encapsulated by the
     <code>odb::sqlite::connection</code> class. For more information
     on <code>sqlite::connection</code>, refer to <a href="#18.3">Section
     18.3, "SQLite Connection and Connection Factory"</a>.</p>

  <h2><a name="18.3">18.3 SQLite Connection and Connection Factory</a></h2>

  <p>The <code>sqlite::connection</code> class has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace sqlite
  {
    class connection: public odb::connection
    {
    public:
      connection (database&amp;, int extra_flags = 0);
      connection (database&amp;, sqlite3*);

      transaction
      begin_immediate ();

      transaction
      begin_exclusive ();

      sqlite3*
      handle ();
    };

    using connection_ptr = details::shared_ptr&lt;connection>;
  }
}
  </pre>

  <p>For more information on the <code>odb::connection</code> interface,
     refer to <a href="#3.6">Section 3.6, "Connections"</a>. The first
     overloaded <code>sqlite::connection</code> constructor opens
     a new SQLite connection. The <code>extra_flags</code> argument can
     be used to specify extra <code>sqlite3_open_v2()</code> flags
     that are combined with the flags specified in the
     <code>sqlite::database</code> constructor. The second constructor
     allows us to create a <code>connection</code> instance by providing
     an already open native SQLite handle. Note that the
     <code>connection</code> instance assumes ownership of this handle.</p>

  <p>The <code>begin_immediate()</code> and <code>begin_exclusive()</code>
     functions allow us to start an immediate and an exclusive SQLite
     transaction on the connection, respectively. Their semantics are
     equivalent to the corresponding functions defined in the
     <code>sqlite::database</code> class (<a href="#18.2">Section 18.2,
     "SQLite Database Class"</a>). The <code>handle()</code> accessor
     returns the SQLite handle corresponding to the connection.</p>

  <p>The <code>sqlite::connection_factory</code> abstract class has the
     following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace sqlite
  {
    class connection_factory
    {
    public:
      virtual void
      database (database&amp;) = 0;

      virtual connection_ptr
      connect () = 0;
    };
  }
}
  </pre>

  <p>The <code>database()</code> function is called when a connection
     factory is associated with a database instance. This happens in
     the <code>odb::sqlite::database</code> class constructors. The
     <code>connect()</code> function is called whenever a database
     connection is requested.</p>

  <p>The four implementations of the <code>connection_factory</code>
     interface provided by the SQLite ODB runtime library are
     <code>serial_connection_factory</code>,
     <code>single_connection_factory</code>,
     <code>new_connection_factory</code>, and
     <code>connection_pool_factory</code>. You will need to include
     the <code>&lt;odb/sqlite/connection-factory.hxx></code>
     header file to make the <code>connection_factory</code> interface
     and these implementation classes available in your application.</p>

  <p>The <code>serial_connection_factory</code> class creates a single
     connection that is expected to be used by an application in a serialized
     manner. For example, a single-threaded application that executes all the
     database operations via the database instance and without dealing with
     multiple connections/transactions would qualify. The
     <code>serial_connection_factory</code> class has the following
     interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace sqlite
  {
    class serial_connection_factory: public connection_factory
    {
    public:
      serial_connection_factory ();

    protected:
      virtual connection_ptr
      create ();
    };
};
  </pre>

  <p>The <code>create()</code> virtual function is called when the
     factory needs to create the connection. By deriving from the
     <code>serial_connection_factory</code> class and overriding this
     function we can implement custom connection establishment
     and configuration.</p>

  <p>The <code>single_connection_factory</code> class creates a
     single connection that is shared between all the threads in
     an application. If the connection is currently not in use,
     then it is returned to the caller. Otherwise, the caller is
     blocked until the connection becomes available. The
     <code>single_connection_factory</code> class has the following
     interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace sqlite
  {
    class single_connection_factory: public connection_factory
    {
    public:
      single_connection_factory ();

    protected:
      class single_connection: public connection
      {
      public:
        single_connection (database&amp;, int extra_flags = 0);
        single_connection (database&amp;, sqlite3*);
      };

      using single_connection_ptr = details::shared_ptr&lt;single_connection>;

      virtual single_connection_ptr
      create ();
    };
};
  </pre>

  <p>The <code>create()</code> virtual function is called when the
     factory needs to create the connection. By deriving from the
     <code>single_connection_factory</code> class and overriding this
     function we can implement custom connection establishment
     and configuration.</p>

  <p>The <code>new_connection_factory</code> class creates a new
     connection whenever one is requested. When a connection is no
     longer needed, it is released and closed. The
     <code>new_connection_factory</code> class has the following
     interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace sqlite
  {
    class new_connection_factory: public connection_factory
    {
    public:
      new_connection_factory ();
    };
};
  </pre>

  <p>The <code>connection_pool_factory</code> class implements a
     connection pool. It has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace sqlite
  {
    class connection_pool_factory: public connection_factory
    {
    public:
      connection_pool_factory (std::size_t max_connections = 0,
                               std::size_t min_connections = 0);

    protected:
      class pooled_connection: public connection
      {
      public:
        pooled_connection (database_type&amp;, int extra_flags = 0);
        pooled_connection (database_type&amp;, sqlite3*);
      };

      using pooled_connection_ptr = details::shared_ptr&lt;pooled_connection>;

      virtual pooled_connection_ptr
      create ();
    };
};
  </pre>

  <p>The <code>max_connections</code> argument in the
     <code>connection_pool_factory</code> constructor specifies the maximum
     number of concurrent connections that this pool factory will
     maintain. Similarly, the <code>min_connections</code> argument
     specifies the minimum number of available connections that
     should be kept open.</p>

  <p>Whenever a connection is requested, the pool factory first
     checks if there is an unused connection that can be returned.
     If there is none, the pool factory checks the
     <code>max_connections</code> value to see if a new connection
     can be created. If the total number of connections maintained
     by the pool is less than this value, then a new connection is
     created and returned. Otherwise, the caller is blocked until
     a connection becomes available.</p>

  <p>When a connection is released, the pool factory first checks
     if there are blocked callers waiting for a connection. If so, then
     one of them is unblocked and is given the connection. Otherwise,
     the pool factory checks whether the total number of connections
     maintained by the pool is greater than the <code>min_connections</code>
     value. If that's the case, the connection is closed. Otherwise, the
     connection is added to the pool of available connections to be
     returned on the next request. In other words, if the number of
     connections maintained by the pool exceeds <code>min_connections</code>
     and there are no callers waiting for a new connection,
     then the pool will close the excess connections.</p>

  <p>If the <code>max_connections</code> value is 0, then the pool will
     create a new connection whenever all of the existing connections
     are in use. If the <code>min_connections</code> value is 0, then
     the pool will never close a connection and instead maintain all
     the connections that were ever created.</p>

  <p>The <code>create()</code> virtual function is called whenever the
     pool needs to create a new connection. By deriving from the
     <code>connection_pool_factory</code> class and overriding this
     function we can implement custom connection establishment
     and configuration.</p>

  <p>By default, connections created by <code>new_connection_factory</code>
     and <code>connection_pool_factory</code> enable the SQLite shared cache
     mode and use the unlock notify functionality to aid concurrency. To
     disable the shared cache mode you can pass the
     <code>SQLITE_OPEN_PRIVATECACHE</code> flag when creating the database
     instance. For more information on the shared cache mode refer to the
     SQLite documentation.</p>

  <p>If you pass <code>NULL</code> as the connection factory to one of the
     <code>database</code> constructors, then the <code>connection_pool_factory</code>
     instance will be created by default with the min and max connections
     values set to <code>0</code>. The following code fragment shows how we
     can pass our own connection factory instance:</p>

  <pre class="cxx">
#include &lt;odb/database.hxx>

#include &lt;odb/sqlite/database.hxx>
#include &lt;odb/sqlite/connection-factory.hxx>

int
main (int argc, char* argv[])
{
  unique_ptr&lt;odb::sqlite::connection_factory> f (
    new odb::sqlite::connection_pool_factory (20));

  unique_ptr&lt;odb::database> db (
    new sqlite::database (argc, argv, false, SQLITE_OPEN_READWRITE, f));
}
  </pre>


  <h2><a name="18.4">18.4 Attached SQLite Databases</a></h2>

  <p>The SQLite database class provides support for attaching additional
     databases to the main database connections using the SQLite <code>ATTACH
     DATABASE</code> statement. Good understanding of the SQLite attached
     database semantics and ODB connection management is strongly recommended
     when using this functionality.</p>

  <p>The relevant part of the SQLite database class interface is shown
     below:</p>

  <pre class="cxx">
namespace odb
{
  namespace sqlite
  {
    class database: public odb::database
    {
    public:

      ...

      database (const connection_ptr&amp;,
                const std::string&amp; name,
                const std::string&amp; schema,
                std::[auto|unique]_ptr&lt;attached_connection_factory> = 0);

      void
      detach ();

      database&amp;
      main_database ();

      const std::string&amp;
      schema () const;
    };
  }
}
  </pre>

  <p>The shown constructor attaches to the specified connection a database
     with the specified name as the specified schema.</p>

  <p>The resulting database instance is referred to as an <em>attached
     database</em> and the connection it returns as an <em>attached
     connection</em> (which is just a proxy for the main connection). Database
     operations executed on the attached database or attached connection are
     automatically translated to refer to the specified schema rather than
     <code>"main"</code>. For uniformity attached databases can also be
     created for the pre-attached <code>"main"</code> and <code>"temp"</code>
     schemas (in this case the <code>name</code> argument can be
     anything).</p>

  <p>The automatic translation of the statements relies on their text having
     references to top-level database entities (tables, indexes, etc)
     qualified with the <code>"main"</code> schema. To achieve this, compile
     your headers with the <code>--schema main</code> option and, if using
     schema migration, with the <code>--schema-version-table
     main.schema_version</code> option. You must also not
     use <code>"main"</code> as an object/table alias in views of native
     statements. For optimal translation performance use 4-character schema
     names.</p>

  <p>The main connection and attached to it databases and connections are all
     meant to be used within the same thread. In particular, the attached
     database holds a counted reference to the main connection which means the
     connection will not be released until all the attached to this connection
     databases are destroyed.</p>

  <p>Note that in this model the attached databases are attached to the main
     connection, not to the (main) database, which mimics the underlying
     semantics of SQLite. An alternative model would have been to notionally
     attach the databases to the main database and under the hood
     automatically attach them to each returned connection. While this may
     seem like a more convenient model in some cases, it is also less
     flexible: the current model allows attaching a different set of databases
     to different connections, attaching them on demand as the transaction
     progresses, etc. Also, the more convenient model can be implemented on
     top of this model by deriving an application-specific database class
     and/or providing custom connection factories.</p>

  <p>Note also that unless the name is a URI with appropriate mode, the
     attached database is opened with the
     <code>SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE</code> flags. In
     particular, if you want just <code>SQLITE_OPEN_READWRITE</code>, then you
     will need to verify the database file existence manually prior to calling
     this constructor.</p>

  <p>Note that attaching/detaching databases within a transaction is only
     supported since SQLite 3.21.0.</p>

  <p>The <code>detach()</code> function detaches the attached database. The
     database is automatically detached on destruction but a failure to detach
     is ignored. To detect such a failure perform explicit detach. For
     uniformity detaching a main database is a no-op.</p>

  <p>The <code>main_database()</code> function returns the main database of an
     attached database. If this database is main, return itself.</p>

  <p>The <code>schema()</code> function returns the schema name under which
     this database was attached or empty if this is the main database.</p>


  <h2><a name="18.5">18.5 SQLite Exceptions</a></h2>

  <p>The SQLite ODB runtime library defines the following SQLite-specific
     exceptions:</p>

  <pre class="cxx">
namespace odb
{
  namespace sqlite
  {
    class forced_rollback: odb::recoverable
    {
    public:
      virtual const char*
      what () const throw ();
    };

    class database_exception: odb::database_exception
    {
    public:
      int
      error () const

      int
      extended_error () const;

      const std::string&amp;
      message () const;

      virtual const char*
      what () const throw ();
    };

    class cli_exception: odb::exception
    {
    public:
      virtual const char*
      what () const throw ();
    };
  }
}
  </pre>

  <p>You will need to include the <code>&lt;odb/sqlite/exceptions.hxx></code>
     header file to make these exceptions available in your application.</p>

  <p>The <code>odb::sqlite::forced_rollback</code> exception is thrown if
     SQLite is forcing the current transaction to roll back. For more
     information on this behavior refer to <a href="#18.6.6">Section 18.6.6,
     "Forced Rollback"</a>.</p>

  <p>The <code>odb::sqlite::database_exception</code> is thrown if
     an SQLite database operation fails. The SQLite-specific error
     information is accessible via the <code>error()</code>,
     <code>extended_error()</code>, and <code>message()</code> functions.
     All this information is also combined and returned in a
     human-readable form by the <code>what()</code> function.</p>

  <p>The <code>odb::sqlite::cli_exception</code> is thrown by the
     command line parsing constructor of the <code>odb::sqlite::database</code>
     class if the SQLite option values are missing or invalid. The
     <code>what()</code> function returns a human-readable description
     of an error.</p>


  <h2><a name="18.6">18.6 SQLite Limitations</a></h2>

  <p>The following sections describe SQLite-specific limitations imposed by
     the current SQLite and ODB runtime versions.</p>

  <h3><a name="18.6.1">18.6.1 Query Result Caching</a></h3>

  <p>SQLite ODB runtime implementation does not perform query result caching
     (<a href="#4.4">Section 4.4, "Query Result"</a>) even when explicitly
     requested. The SQLite API supports interleaving execution of multiple
     prepared statements on a single connection. As a result, with SQLite, it
     is possible to have multiple uncached results and calls to other database
     functions do not invalidate them. The only limitation of the uncached
     SQLite results is the unavailability of the <code>result::size()</code>
     function. If you call this function on an SQLite query result, then
     the <code>odb::result_not_cached</code> exception
     (<a href="#3.14">Section 3.14, "ODB Exceptions"</a>) is always
     thrown. Future versions of the SQLite ODB runtime library may add support
     for result caching.</p>

  <h3><a name="18.6.2">18.6.2 Automatic Assignment of Object Ids</a></h3>

  <p>Due to SQLite API limitations, every automatically assigned object id
     (<a href="#14.4.2">Section 14.4.2, "<code>auto</code>"</a>) should have
     the <code>INTEGER</code> SQLite type. While SQLite will treat other
     integer type names (such as <code>INT</code>, <code>BIGINT</code>, etc.)
     as <code>INTEGER</code>, automatic id assignment will not work. By default,
     ODB maps all C++ integral types to <code>INTEGER</code>. This means that
     the only situation that requires consideration is the assignment of a
     custom database type using the <code>db&nbsp;type</code> pragma
     (<a href="#14.4.3">Section 14.4.3, "<code>type</code>"</a>). For
     example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  //#pragma db id auto type("INT")     // Will not work.
  //#pragma db id auto type("INTEGER") // Ok.
  #pragma db id auto                   // Ok, Mapped to INTEGER.
  unsigned int id_;
};
  </pre>

  <h3><a name="18.6.3">18.6.3 Foreign Key Constraints</a></h3>

  <p>By default the SQLite ODB runtime enables foreign key constraints
     checking (<code>PRAGMA foreign_keys=ON</code>). You can disable foreign
     keys by passing <code>false</code> as the <code>foreign_keys</code>
     argument to one of the <code>odb::sqlite::database</code> constructors.
     Foreign keys will also be disabled if the SQLite library is built without
     support for foreign keys (<code>SQLITE_OMIT_FOREIGN_KEY</code> and
     <code>SQLITE_OMIT_TRIGGER</code> macros) or if you are using
     an SQLite version prior to 3.6.19, which does not support foreign
     key constraints checking.</p>

  <p>If foreign key constraints checking is disabled or not available,
     then inconsistencies in object relationships will not be detected.
     Furthermore, using the <code>erase_query()</code> function
     (<a href="#3.11">Section 3.11, "Deleting Persistent Objects"</a>)
     to delete persistent objects that contain containers will not work
     correctly. Container data for such objects will not be deleted.</p>

  <p>When foreign key constraints checking is enabled, then you may
     get the "foreign key constraint failed" error while re-creating the
     database schema. This error is due to bugs in the SQLite DDL foreign
     keys support. The recommended work-around for this problem is to
     temporarily disable foreign key constraints checking while
     re-creating the schema. The following code fragment shows how
     this can be done:</p>

  <pre class="cxx">
#include &lt;odb/connection.hxx>
#include &lt;odb/transaction.hxx>
#include &lt;odb/schema-catalog.hxx>

odb::database&amp; db = ...

{
  odb::connection_ptr c (db.connection ());

  c->execute ("PRAGMA foreign_keys=OFF");

  odb::transaction t (c->begin ());
  odb::schema_catalog::create_schema (db);
  t.commit ();

  c->execute ("PRAGMA foreign_keys=ON");
}
  </pre>

  <p>Finally, ODB assumes the standard SQL behavior which requires
     that foreign key constraints checking is deferred until the
     transaction is committed. Default SQLite behavior is to check such
     constraints immediately. As a result, when used with ODB, a custom
     database schema that defines foreign key constraints may need to
     declare such constraints as <code>DEFERRABLE INITIALLY DEFERRED</code>,
     as shown in the following example. By default, schemas generated by
     the ODB compiler meet this requirement automatically.</p>

  <pre class="sql">
CREATE TABLE Employee (
  ...
  employer INTEGER REFERENCES Employer(id)
           DEFERRABLE INITIALLY DEFERRED);
  </pre>

  <p>You can override the default behavior and instruct the ODB
     compiler to generate non-deferrable foreign keys by specifying
     the <code>--fkeys-deferrable-mode not_deferrable</code> ODB
     compiler option. Note, however, that in this case the order in
     which you persist, update, and erase objects within a transaction
     becomes important.</p>

  <h3><a name="18.6.4">18.6.4 Constraint Violations</a></h3>

  <p>Due to the granularity of the SQLite error codes, it is impossible
     to distinguish between the duplicate primary key and other constraint
     violations. As a result, when making an object persistent, the SQLite
     ODB runtime will translate all constraint violation errors to the
     <code>object_already_persistent</code> exception (<a href="#3.14">Section
     3.14, "ODB Exceptions"</a>).</p>

  <h3><a name="18.6.5">18.6.5 Sharing of Queries</a></h3>

  <p>As discussed in <a href="#4.3">Section 4.3, "Executing a Query"</a>, a
     query instance that does not have any by-reference parameters is
     immutable and can be shared between multiple threads without
     synchronization. Currently, the SQLite ODB runtime does not support this
     functionality. Future versions of the library will remove this
     limitation.</p>

  <h3><a name="18.6.6">18.6.6 Forced Rollback</a></h3>

  <p>In SQLite 3.7.11 or later, if one of the connections participating in
     the shared cache rolls back a transaction, then ongoing transactions
     on other connections in the shared cache may also be forced to roll back.
     An example of such behavior would be a read-only transaction that is
     forced to roll back while iterating over the query result because another
     transaction on another connection was rolled back.</p>

  <p>If a transaction is being forced to roll back by SQLite, then ODB
     throws <code>odb::sqlite::forced_rollback</code>
     (<a href="#18.5">Section 18.5, "SQLite Exceptions"</a>) which is
     a recoverable exception (<a href="#3.7">3.7 Error Handling and
     Recovery</a>). As a result, the recommended way to handle this
     exception is to re-execute the affected transaction.</p>

  <h3><a name="18.6.7">18.6.7 Database Schema Evolution</a></h3>

  <p>From the list of schema migration changes supported by ODB
     (<a href="#13.2">Section 13.2, "Schema Migration"</a>), the
     following are not supported by SQLite:</p>

  <ul class="list">
    <li>drop column</li>
    <li>alter column, set <code>NULL</code>/<code>NOT NULL</code></li>
    <li>add foreign key</li>
    <li>drop foreign key</li>
  </ul>

  <p>The biggest problem is the lack of support for dropping columns.
     This means that it would be impossible to delete a data member
     in a persistent class. To work around this limitation ODB
     implements <em>logical delete</em> for columns that allow
     <code>NULL</code> values. In this case, instead of dropping
     the column (in the post-migration stage), the schema migration
     statements will automatically reset this column in all the
     existing rows to <code>NULL</code>. Any new rows that are
     inserted later will also automatically have this column set
     to <code>NULL</code> (unless the column specifies a default
     value).</p>

  <p>Since it is also impossible to change the column's
     <code>NULL</code>/<code>NOT NULL</code> attribute after it
     has been added, to make schema evolution support usable in
     SQLite, all the columns should be added as <code>NULL</code>
     even if semantically they should not allow <code>NULL</code>
     values. We should also normally refrain from assigning
     default values to columns (<a href="#14.4.7">Section 14.4.7,
     <code>default</code></a>), unless the space overhead of
     a default value is not a concern. Explicitly making all
     the data members <code>NULL</code> would be burdensome
     and ODB provides the <code>--sqlite-override-null</code>
     command line option that forces all the columns, even those
     that were explicitly marked <code>NOT NULL</code>, to be
     <code>NULL</code> in SQLite.</p>

  <p>SQLite only supports adding foreign keys as part of the
     column addition. As a result, we can only add a new
     data member of an object pointer type if it points
     to an object with a simple (single-column) object id.</p>

  <p>SQLite also doesn't support dropping foreign keys.
     Leaving a foreign key around works well with logical
     delete unless we also want to delete the pointed-to
     object. In this case we will have to leave an
     empty table corresponding to the pointed-to object
     around. An alternative would be to make a copy of the
     pointing object without the object pointer, migrate the
     data, and then delete both the old pointing and the
     pointed-to objects. Since this will result in dropping
     the pointing table, the foreign key will be dropped
     as well. Yet another, more radical, solution to this
     problem is to disable foreign keys checking altogether
     (see the <code>foreign_keys</code> SQLite pragma).</p>

  <p>To summarize, to make schema evolution support usable
     in SQLite we should pass the <code>--sqlite-override-null</code>
     option when compiling our persistent classes and also refrain
     from assigning default values to data members. Note also that
     this has to be done from the start so that every column is added
     as <code>NULL</code> and therefore can be logically deleted later.
     In particular, you cannot add the <code>--sqlite-override-null</code>
     option when you realize you need to delete a data member. At this
     point it is too late since the column has already been added
     as <code>NOT NULL</code> in existing databases. We should also
     avoid composite object ids if we are planning to use object
     relationships.</p>

  <h2><a name="18.7">18.7 SQLite Index Definitions</a></h2>

  <p>When the <code>index</code> pragma (<a href="#14.7">Section 14.7,
     "Index Definition Pragmas"</a>) is used to define an SQLite index,
     the <code>type</code> clause specifies the index type (for example,
     <code>UNIQUE</code>) while the <code>method</code> and
     <code>options</code> clauses are not used. The column options
     can be used to specify collations and the sort order. For example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  std::string name_;

  #pragma db index member(name_, "COLLATE binary DESC")
};
  </pre>

  <p>Index names in SQLite are database-global. To avoid name clashes,
     ODB automatically prefixes each index name with the table name on
     which it is defined.</p>

  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="19">19 PostgreSQL Database</a></h1>

  <p>To generate support code for the PostgreSQL database you will need
     to pass the "<code>--database&nbsp;pgsql</code>"
     (or "<code>-d&nbsp;pgsql</code>") option to the ODB compiler.
     Your application will also need to link to the PostgreSQL ODB runtime
     library (<code>libodb-pgsql</code>). All PostgreSQL-specific ODB
     classes are defined in the <code>odb::pgsql</code> namespace.</p>

  <p>ODB utilizes prepared statements extensively. Support for prepared
     statements was added in PostgreSQL version 7.4 with the introduction
     of the messaging protocol version 3.0. For this reason, ODB supports
     only PostgreSQL version 7.4 and later.</p>

  <h2><a name="19.1">19.1 PostgreSQL Type Mapping</a></h2>

  <p>The following table summarizes the default mapping between basic
     C++ value types and PostgreSQL database types. This mapping can be
     customized on the per-type and per-member basis using the ODB
     Pragma Language (<a href="#14">Chapter 14, "ODB Pragma
     Language"</a>).</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>C++ Type</th>
      <th>PostgreSQL Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>bool</code></td>
      <td><code>BOOLEAN</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>char</code></td>
      <td><code>CHAR(1)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>signed char</code></td>
      <td><code>SMALLINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned char</code></td>
      <td><code>SMALLINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>short</code></td>
      <td><code>SMALLINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned short</code></td>
      <td><code>SMALLINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>int</code></td>
      <td><code>INTEGER</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned int</code></td>
      <td><code>INTEGER</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>long</code></td>
      <td><code>BIGINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned long</code></td>
      <td><code>BIGINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>long long</code></td>
      <td><code>BIGINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned long long</code></td>
      <td><code>BIGINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>float</code></td>
      <td><code>REAL</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>double</code></td>
      <td><code>DOUBLE PRECISION</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>std::string</code></td>
      <td><code>TEXT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>char[N]</code></td>
      <td><code>VARCHAR(N-1)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>
  </table>

  <p>It is possible to map the <code>char</code> C++ type to an integer
     database type (for example, <code>SMALLINT</code>) using the
     <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section 14.4.3,
     "<code>type</code>"</a>).</p>

  <p>Additionally, by default, C++ enums and C++11 enum classes are
     automatically mapped to the PostgreSQL types corresponding to their
     underlying integral types (see table above). The default
     <code>NULL</code> semantics is <code>NOT NULL</code>. For
     example:</p>

  <pre class="cxx">
enum color {red, green, blue};
enum class taste: unsigned char
{
  bitter = 1,
  sweet,
  sour = 4,
  salty
};

#pragma db object
class object
{
  ...

  color color_; // Automatically mapped to INTEGER.
  taste taste_; // Automatically mapped to SMALLINT.
};
  </pre>

  <p>Note also that because PostgreSQL does not support unsigned integers,
     the <code>unsigned&nbsp;short</code>, <code>unsigned&nbsp;int</code>, and
     <code>unsigned&nbsp;long</code>/<code>unsigned&nbsp;long&nbsp;long</code> C++ types
     are by default mapped to the <code>SMALLINT</code>, <code>INTEGER</code>,
     and <code>BIGINT</code> PostgreSQL types, respectively. The sign bit
     of the value stored by the database for these types will contain
     the most significant bit of the actual unsigned value being
     persisted.</p>

  <p>It is also possible to add support for additional PostgreSQL types,
     such as <code>NUMERIC</code>, geometry types, <code>XML</code>,
     <code>JSON</code>, enumeration types, composite types, arrays,
     geospatial types, and the key-value store (<code>HSTORE</code>).
     For more information, refer to <a href="#14.8">Section 14.8,
     "Database Type Mapping Pragmas"</a>.</p>

  <h3><a name="19.1.1">19.1.1 String Type Mapping</a></h3>

  <p>The PostgreSQL ODB runtime library provides support for mapping the
     <code>std::string</code>, <code>char[N]</code>, and
     <code>std::array&lt;char, N></code> types to the PostgreSQL
     <code>CHAR</code>, <code>VARCHAR</code>, and <code>TEXT</code>
     types. However, these mappings are not enabled by default (in
     particular, by default, <code>std::array</code> will be treated
     as a container). To enable the alternative mappings for these
     types we need to specify the database type explicitly using the
     <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section 14.4.3,
     "<code>type</code>"</a>), for example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  #pragma db type("CHAR(2)")
  char state_[2];

  #pragma db type("VARCHAR(128)")
  std::string name_;
};
  </pre>

  <p>Alternatively, this can be done on the per-type basis, for example:</p>

  <pre class="cxx">
#pragma db value(std::string) type("VARCHAR(128)")

#pragma db object
class object
{
  ...

  std::string name_; // Mapped to VARCHAR(128).
};
  </pre>

  <p>The <code>char[N]</code> and <code>std::array&lt;char, N></code> values
     may or may not be zero-terminated. When extracting such values from the
     database, ODB will append the zero terminator if there is enough
     space.</p>

  <h3><a name="19.1.2">19.1.2 Binary Type and <code>UUID</code> Mapping</a></h3>

  <p>The PostgreSQL ODB runtime library provides support for mapping the
     <code>std::vector&lt;char></code>,
     <code>std::vector&lt;unsigned&nbsp;char></code>,
     <code>char[N]</code>, <code>unsigned&nbsp;char[N]</code>,
     <code>std::array&lt;char, N></code>, and
     <code>std::array&lt;unsigned char, N></code> types to the PostgreSQL
     <code>BYTEA</code> type. There is also support for mapping the
     <code>char[16]</code> array to the PostgreSQL <code>UUID</code> type.
     However, these mappings are not enabled by default (in particular, by
     default, <code>std::vector</code> and <code>std::array</code> will be
     treated as containers). To enable the alternative mappings for these
     types we need to specify the database type explicitly using the
     <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section 14.4.3,
     "<code>type</code>"</a>), for example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  #pragma db type("UUID")
  char uuid_[16];

  #pragma db type("BYTEA")
  std::vector&lt;char> buf_;

  #pragma db type("BYTEA")
  unsigned char data_[256];
};
  </pre>

  <p>Alternatively, this can be done on the per-type basis, for example:</p>

  <pre class="cxx">
using buffer = std::vector&lt;char>;
#pragma db value(buffer) type("BYTEA")

#pragma db object
class object
{
  ...

  buffer buf_; // Mapped to BYTEA.
};
  </pre>

  <p>Note also that in native queries (<a href="#4">Chapter 4, "Querying
     the Database"</a>) <code>char[N]</code> and
     <code>std::array&lt;char, N></code> parameters are by default passed
     as a string rather than a binary. To pass such parameters as a binary,
     we need to specify the database type explicitly in the
     <code>_val()</code>/<code>_ref()</code> calls. Note also that we
     don't need to do this for the integrated queries, for example:</p>

  <pre class="cxx">
char u[16] = {...};

db.query&lt;object> ("uuid = " + query::_val&lt;odb::pgsql::id_uuid> (u));
db.query&lt;object> ("buf = " + query::_val&lt;odb::pgsql::id_bytea> (u));
db.query&lt;object> (query::uuid == query::_ref (u));
  </pre>

  <h2><a name="19.2">19.2 PostgreSQL Database Class</a></h2>

  <p>The PostgreSQL <code>database</code> class has the following
     interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace pgsql
  {
    class database: public odb::database
    {
    public:
      database (const std::string&amp; user,
                const std::string&amp; password,
                const std::string&amp; db,
                const std::string&amp; host = "",
                unsigned int port = 0,
                const std::string&amp; extra_conninfo = "",
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      database (const std::string&amp; user,
                const std::string&amp; password,
                const std::string&amp; db,
                const std::string&amp; host,
                const std::string&amp; socket_ext,
                const std::string&amp; extra_conninfo = "",
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      database (const std::string&amp; conninfo,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      database (int&amp; argc,
                char* argv[],
                bool erase = false,
                const std::string&amp; extra_conninfo = "",
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      static void
      print_usage (std::ostream&amp;);

    public:
      const std::string&amp;
      user () const;

      const std::string&amp;
      password () const;

      const std::string&amp;
      db () const;

      const std::string&amp;
      host () const;

      unsigned int
      port () const;

      const std::string&amp;
      socket_ext () const;

      const std::string&amp;
      extra_conninfo () const;

      const std::string&amp;
      conninfo () const;

    public:
      connection_ptr
      connection ();
    };
  }
}
  </pre>

  <p>You will need to include the <code>&lt;odb/pgsql/database.hxx></code>
     header file to make this class available in your application.</p>

  <p>The overloaded <code>database</code> constructors allow us to specify
     the PostgreSQL database parameters that should be used when connecting
     to the database. The <code>port</code> argument in the first constructor
     is an integer value specifying the TCP/IP port number to connect to. A
     zero port number indicates that the default port should be used.
     The <code>socket_ext</code> argument in the second constructor is a
     string value specifying the UNIX-domain socket file name extension.</p>

  <p>The third constructor allows us to specify all the database parameters
     as a single <code>conninfo</code> string. All other constructors
     accept additional database connection parameters as the
     <code>extra_conninfo</code> argument. For more information
     about the format of the <code>conninfo</code> string, refer to
     the <code>PQconnectdb()</code> function description in the PostgreSQL
     documentation. In the case of <code>extra_conninfo</code>, all the
     database parameters provided in this string will take precedence
     over those explicitly specified with other constructor arguments.</p>

  <p>The last constructor extracts the database parameters
     from the command line. The following options are recognized:</p>

  <pre class="terminal">
  --user &lt;login> | --username &lt;login>
  --password &lt;password>
  --database &lt;name> | --dbname &lt;name>
  --host &lt;host>
  --port &lt;integer>
  --options-file &lt;file>
  </pre>

  <p>The <code>--options-file</code> option allows us to specify some
     or all of the database options in a file with each option appearing
     on a separate line followed by a space and an option value.</p>

  <p>If the <code>erase</code> argument to this constructor is true,
     then the above options are removed from the <code>argv</code>
     array and the <code>argc</code> count is updated accordingly.
     This is primarily useful if your application accepts other
     options or arguments and you would like to get the PostgreSQL
     options out of the <code>argv</code> array.</p>

  <p>This constructor throws the <code>odb::pgsql::cli_exception</code>
     exception if the PostgreSQL option values are missing or invalid.
     See section <a href="#19.4">Section 19.4, "PostgreSQL Exceptions"</a>
     for more information on this exception.</p>

  <p>The static <code>print_usage()</code> function prints the list of options
     with short descriptions that are recognized by this constructor.</p>

  <p>The last argument to all of the constructors is a pointer to the
     connection factory. In C++98/03, it is <code>std::auto_ptr</code> while
     in C++11 <code>std::unique_ptr</code> is used instead. If we pass a
     non-<code>NULL</code> value, the database instance assumes ownership
     of the factory instance. The connection factory interface as well as
     the available implementations are described in the next section.</p>

  <p>The set of accessor functions following the constructors allows us
     to query the parameters of the <code>database</code> instance. Note that
     the <code>conninfo()</code> accessor returns a complete
     <code>conninfo</code> string which includes parameters that were
     explicitly specified with the various constructor arguments, as well as
     the extra parameters passed in the <code>extra_conninfo</code> argument.
     The <code>extra_conninfo()</code> accessor will return the
     <code>conninfo</code> string as passed in the <code>extra_conninfo</code>
     argument.</p>

  <p>The <code>connection()</code> function returns a pointer to the
     PostgreSQL database connection encapsulated by the
     <code>odb::pgsql::connection</code> class. For more information
     on <code>pgsql::connection</code>, refer to <a href="#19.3">Section
     19.3, "PostgreSQL Connection and Connection Factory"</a>.</p>

  <h2><a name="19.3">19.3 PostgreSQL Connection and Connection Factory</a></h2>

  <p>The <code>pgsql::connection</code> class has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace pgsql
  {
    class connection: public odb::connection
    {
    public:
      connection (database&amp;);
      connection (database&amp;, PGconn*);

      PGconn*
      handle ();
    };

    using connection_ptr = details::shared_ptr&lt;connection>;
  }
}
  </pre>

  <p>For more information on the <code>odb::connection</code> interface,
     refer to <a href="#3.6">Section 3.6, "Connections"</a>. The first
     overloaded <code>pgsql::connection</code> constructor establishes a
     new PostgreSQL connection. The second constructor allows us to create
     a <code>connection</code> instance by providing an already connected
     native PostgreSQL handle. Note that the <code>connection</code>
     instance assumes ownership of this handle. The <code>handle()</code>
     accessor returns the PostgreSQL handle corresponding to the connection.</p>

  <p>The <code>pgsql::connection_factory</code> abstract class has the
     following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace pgsql
  {
    class connection_factory
    {
    public:
      virtual void
      database (database&amp;) = 0;

      virtual connection_ptr
      connect () = 0;
    };
  }
}
  </pre>

  <p>The <code>database()</code> function is called when a connection
     factory is associated with a database instance. This happens in
     the <code>odb::pgsql::database</code> class constructors. The
     <code>connect()</code> function is called whenever a database
     connection is requested.</p>

  <p>The two implementations of the <code>connection_factory</code>
     interface provided by the PostgreSQL ODB runtime are
     <code>new_connection_factory</code> and
     <code>connection_pool_factory</code>. You will need to include
     the <code>&lt;odb/pgsql/connection-factory.hxx></code>
     header file to make the <code>connection_factory</code> interface
     and these implementation classes available in your application.</p>

  <p>The <code>new_connection_factory</code> class creates a new
     connection whenever one is requested. When a connection is no
     longer needed, it is released and closed. The
     <code>new_connection_factory</code> class has the following
     interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace pgsql
  {
    class new_connection_factory: public connection_factory
    {
    public:
      new_connection_factory ();
    };
};
  </pre>

  <p>The <code>connection_pool_factory</code> class implements a
     connection pool. It has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace pgsql
  {
    class connection_pool_factory: public connection_factory
    {
    public:
      connection_pool_factory (std::size_t max_connections = 0,
                               std::size_t min_connections = 0);

    protected:
      class pooled_connection: public connection
      {
      public:
        pooled_connection (database_type&amp;);
        pooled_connection (database_type&amp;, PGconn*);
      };

      using pooled_connection_ptr = details::shared_ptr&lt;pooled_connection>;

      virtual pooled_connection_ptr
      create ();
    };
};
  </pre>

  <p>The <code>max_connections</code> argument in the
     <code>connection_pool_factory</code> constructor specifies the maximum
     number of concurrent connections that this pool factory will
     maintain. Similarly, the <code>min_connections</code> argument
     specifies the minimum number of available connections that
     should be kept open.</p>

  <p>Whenever a connection is requested, the pool factory first
     checks if there is an unused connection that can be returned.
     If there is none, the pool factory checks the
     <code>max_connections</code> value to see if a new connection
     can be created. If the total number of connections maintained
     by the pool is less than this value, then a new connection is
     created and returned. Otherwise, the caller is blocked until
     a connection becomes available.</p>

  <p>When a connection is released, the pool factory first checks
     if there are blocked callers waiting for a connection. If so, then
     one of them is unblocked and is given the connection. Otherwise,
     the pool factory checks whether the total number of connections
     maintained by the pool is greater than the <code>min_connections</code>
     value. If that's the case, the connection is closed. Otherwise, the
     connection is added to the pool of available connections to be
     returned on the next request. In other words, if the number of
     connections maintained by the pool exceeds <code>min_connections</code>
     and there are no callers waiting for a new connection,
     the pool will close the excess connections.</p>

  <p>If the <code>max_connections</code> value is 0, then the pool will
     create a new connection whenever all of the existing connections
     are in use. If the <code>min_connections</code> value is 0, then
     the pool will never close a connection and instead maintain all
     the connections that were ever created.</p>

  <p>The <code>create()</code> virtual function is called whenever the
     pool needs to create a new connection. By deriving from the
     <code>connection_pool_factory</code> class and overriding this
     function we can implement custom connection establishment
     and configuration.</p>

  <p>If you pass <code>NULL</code> as the connection factory to one of the
     <code>database</code> constructors, then the
     <code>connection_pool_factory</code> instance will be created by default
     with the min and max connections values set to <code>0</code>. The
     following code fragment shows how we can pass our own connection factory
     instance:</p>

  <pre class="cxx">
#include &lt;odb/database.hxx>

#include &lt;odb/pgsql/database.hxx>
#include &lt;odb/pgsql/connection-factory.hxx>

int
main (int argc, char* argv[])
{
  unique_ptr&lt;odb::pgsql::connection_factory> f (
    new odb::pgsql::connection_pool_factory (20));

  unique_ptr&lt;odb::database> db (
    new pgsql::database (argc, argv, false, "", f));
}
  </pre>

  <h2><a name="19.4">19.4 PostgreSQL Exceptions</a></h2>

  <p>The PostgreSQL ODB runtime library defines the following
     PostgreSQL-specific exceptions:</p>

  <pre class="cxx">
namespace odb
{
  namespace pgsql
  {
    class database_exception: odb::database_exception
    {
    public:
      const std::string&amp;
      message () const;

      const std::string&amp;
      sqlstate () const;

      virtual const char*
      what () const throw ();
    };

    class cli_exception: odb::exception
    {
    public:
      virtual const char*
      what () const throw ();
    };
  }
}
  </pre>

  <p>You will need to include the <code>&lt;odb/pgsql/exceptions.hxx></code>
     header file to make these exceptions available in your application.</p>

  <p>The <code>odb::pgsql::database_exception</code> is thrown if
     a PostgreSQL database operation fails. The PostgreSQL-specific error
     information is accessible via the <code>message()</code> and
     <code>sqlstate()</code> functions. All this information is also
     combined and returned in a human-readable form by the <code>what()</code>
     function.</p>

  <p>The <code>odb::pgsql::cli_exception</code> is thrown by the
     command line parsing constructor of the <code>odb::pgsql::database</code>
     class if the PostgreSQL option values are missing or invalid. The
     <code>what()</code> function returns a human-readable description
     of an error.</p>

  <h2><a name="19.5">19.5 PostgreSQL Limitations</a></h2>

  <p>The following sections describe PostgreSQL-specific limitations imposed
     by the current PostgreSQL and ODB runtime versions.</p>

  <h3><a name="19.5.1">19.5.1 Query Result Caching</a></h3>

  <p>The PostgreSQL ODB runtime implementation will always return a
     cached query result (<a href="#4.4">Section 4.4, "Query Result"</a>)
     even when explicitly requested not to. This is a limitation of the
     PostgreSQL client library (<code>libpq</code>) which does not
     support uncached (streaming) query results.</p>

  <h3><a name="19.5.2">19.5.2 Foreign Key Constraints</a></h3>

  <p>ODB assumes the standard SQL behavior which requires that
     foreign key constraints checking is deferred until the
     transaction is committed. Default PostgreSQL behavior is
     to check such constraints immediately. As a result, when
     used with ODB, a custom database schema that defines foreign
     key constraints may need to declare such constraints as
     <code>INITIALLY DEFERRED</code>, as shown in the following example.
     By default, schemas generated by the ODB compiler meet this requirement
     automatically.</p>

  <pre class="sql">
CREATE TABLE Employee (
  ...
  employer BIGINT REFERENCES Employer(id) INITIALLY DEFERRED);
  </pre>

  <p>You can override the default behavior and instruct the ODB
     compiler to generate non-deferrable foreign keys by specifying
     the <code>--fkeys-deferrable-mode not_deferrable</code> ODB
     compiler option. Note, however, that in this case the order in
     which you persist, update, and erase objects within a transaction
     becomes important.</p>

  <h3><a name="19.5.3">19.5.3 Unique Constraint Violations</a></h3>

  <p>Due to the granularity of the PostgreSQL error codes, it is impossible
     to distinguish between the duplicate primary key and other unique
     constraint violations. As a result, when making an object persistent,
     the PostgreSQL ODB runtime will translate all unique constraint violation
     errors to the <code>object_already_persistent</code> exception
     (<a href="#3.14">Section 3.14, "ODB Exceptions"</a>).</p>

  <h3><a name="19.5.4">19.5.4 Date-Time Format</a></h3>

  <p>ODB expects the PostgreSQL server to use integers as a binary
     format for the date-time types, which is the default for most
     PostgreSQL configurations. When creating a connection, ODB
     examines the <code>integer_datetimes</code> PostgreSQL server
     parameter and if it is <code>false</code>,
     <code>odb::pgsql::database_exception</code> is thrown. You may
     check the value of this parameter for your server by executing
     the following SQL query:</p>

  <pre class="sql">
SHOW integer_datetimes
  </pre>

  <h3><a name="19.5.5">19.5.5 Timezones</a></h3>

  <p>ODB does not currently natively support the PostgreSQL date-time types
     with timezone information. However, these types can be accessed by
     mapping them to one of the natively supported types, as discussed
     in <a href="#14.8">Section 14.8, "Database Type Mapping Pragmas"</a>.</p>

  <h3><a name="19.5.6">19.5.6 <code>NUMERIC</code> Type Support</a></h3>

  <p>Support for the PostgreSQL <code>NUMERIC</code> type is limited
     to providing a binary buffer containing the binary representation
     of the value. For more information on the binary format used to
     store <code>NUMERIC</code> values refer to the PostgreSQL
     documentation. An alternative approach to accessing <code>NUMERIC</code>
     values is to map this type to one of the natively supported
     ones, as discussed in <a href="#14.8">Section 14.8, "Database
     Type Mapping Pragmas"</a>.</p>

  <h3><a name="19.5.7">19.5.7 Bulk Operations Support</a></h3>

  <p>Support for bulk operations (<a href="#15.3">Section 15.3, "Bulk
     Database Operations"</a>) requires PostgreSQL client library
     (<code>libpq</code>) version 14 or later and PostgreSQL server
     version 7.4 or later.</p>


  <h2><a name="19.6">19.6 PostgreSQL Index Definitions</a></h2>

  <p>When the <code>index</code> pragma (<a href="#14.7">Section 14.7,
     "Index Definition Pragmas"</a>) is used to define a PostgreSQL index,
     the <code>type</code> clause specifies the index type (for example,
     <code>UNIQUE</code>), the <code>method</code> clause specifies the
     index method (for example, <code>BTREE</code>, <code>HASH</code>,
     <code>GIN</code>, etc.), and the <code>options</code> clause
     specifies additional index options, such as storage parameters,
     table spaces, and the <code>WHERE</code> predicate. To support
     the definition of concurrent indexes, the <code>type</code>
     clause can end with the word <code>CONCURRENTLY</code> (upper and
     lower cases are recognized). The column options can be used to
     specify collations, operator classes, and the sort order. For example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  std::string name_;

  #pragma db index                            \
             type("UNIQUE CONCURRENTLY")      \
             method("HASH")                   \
             member(name_, "DESC")            \
             options("WITH(FILLFACTOR = 80)")
};
  </pre>

  <p>Index names in PostgreSQL are schema-global. To avoid name clashes,
     ODB automatically prefixes each index name with the table name on
     which it is defined.</p>


  <h2><a name="19.7">19.7 PostgreSQL Stored Procedures and Functions</a></h2>

  <p>ODB native views (<a href="#10.6">Section 10.6, "Native Views"</a>) can
     be used to call PostgreSQL stored procedures and functions. For example,
     assuming we are using the <code>person</code> class
     from <a href="#2">Chapter 2, "Hello World Example"</a> (and the
     corresponding <code>person</code> table), we can create a stored
     function that given the min and max ages returns some information about
     all the people in that range:</p>

  <pre class="sql">
CREATE FUNCTION
person_range (IN min_age INTEGER,
              IN max_age INTEGER,
              OUT age SMALLINT,
              OUT first TEXT,
              OUT last TEXT)
RETURNS SETOF RECORD AS $$
  SELECT age, first, last
    FROM person
    WHERE age >= min_age AND age &lt;= max_age;
$$ LANGUAGE SQL STABLE;
  </pre>

  <p>Given the above stored function we can then define an ODB view that can
     be used to call it and retrieve its result:</p>

  <pre class="cxx">
#pragma db view query("/*CALL*/ SELECT * FROM person_range((?))")
struct person_range
{
  unsigned short age;
  std::string first;
  std::string last;
};
  </pre>

  <p>Notice the special <code>/*CALL*/</code> prefix: because PostgreSQL uses
     ordinary <code>SELECT</code> queries to call functions, we need to
     communicate to ODB that the query is actually a function call.</p>

  <p>The following example shows how we can use the above view to
     print the list of people in a specific age range:</p>

  <pre class="cxx">
using query = odb::query&lt;person_range>;
using result = odb::result&lt;person_range>;

transaction t (db.begin ());

result r (
  db.query&lt;person_range> (
    query::_val (1) + "," + query::_val (18)));

for (result::iterator i (r.begin ()); i != r.end (); ++i)
  cerr &lt;&lt; i->first &lt;&lt; " " &lt;&lt; i->last &lt;&lt; " " &lt;&lt; i->age &lt;&lt; endl;

t.commit ();
  </pre>

  <p>Note that as with all native views, the order and types of data members
     must match those returned by the stored function or procedure.</p>

  <p>In the above example, the stored function returned a set of rows. Other
     common cases are functions that return a single row, a single value, or
     nothing (<code>VOID</code>). An example of a function returning a single
     row via the <code>OUT</code> parameters:</p>

  <pre class="sql">
CREATE FUNCTION
person_age_range (IN last_name TEXT,
                  OUT min_age SMALLINT,
                  OUT max_age SMALLINT)
AS $$
  SELECT min(age), max(age)
    FROM person
    WHERE last = last_name;
$$ LANGUAGE SQL STABLE;
  </pre>

  <pre class="cxx">
#pragma db view query("/*CALL*/ SELECT * FROM person_age_range((?))")
struct person_age_range
{
  unsigned short min_age;
  unsigned short max_age;
};
  </pre>

  <pre class="cxx">
using query = odb::query&lt;person_age_range>;

person_age_range r (
  db.query_value&lt;person_age_range> (
    query::_val ("Doe")));
  </pre>

  <p>An example of a function returning a single value:</p>

  <pre class="sql">
CREATE FUNCTION
person_count ()
RETURNS BIGINT AS $$
  SELECT count(id)
    FROM person;
$$ LANGUAGE SQL STABLE;
  </pre>

  <pre class="cxx">
#pragma db view query("/*CALL*/ SELECT * FROM person_count()")
struct person_count
{
  unsigned long long count;
};
  </pre>

  <pre class="cxx">
unsigned long long count (db.query_value&lt;person_count> ().count);
  </pre>

  <p>Finally, a function that returns <code>VOID</code>:</p>

  <pre class="sql">
CREATE FUNCTION
person_increment_age (IN min_age INTEGER,
                      IN max_age INTEGER)
RETURNS VOID AS $$
  UPDATE person
    SET age = age + 1
    WHERE age >= min_age AND age &lt;= max_age;
$$ LANGUAGE SQL;
  </pre>

  <pre class="cxx">
#pragma db view query("/*CALL*/ SELECT * FROM person_increment_age((?))")
struct person_increment_age {};
  </pre>

  <pre class="cxx">
using query = odb::query&lt;person_increment_age>;

db.query_value&lt;person_increment_age> (
  query::_val (1) + ","  +
  query::_val (18));
  </pre>

 <p>We can reuse a single view to call several functions (or procedure; see
    below) that return <code>VOID</code> or, more generally, return the same
    result, by specifying the function (or procedure) at the call site rather
    than in the view definition. For example:</p>

  <pre class="cxx">
#pragma db view
struct no_result {};

using query = odb::query&lt;no_result>;

db.query_one&lt;no_result> (
  "/*CALL*/ SELECT * FROM person_increment_age(" +
  query::_val (1) + "," +
  query::_val (18) + ")");
  </pre>

  <p>In contrast to functions, stored procedures in PostgreSQL are called with
     the <code>CALL</code> statement. They can only return a single set of
     values via the <code>OUT</code> parameters. If there are no output
     parameters, then a stored procedure doesn't return anything. For
     example:</p>

  <pre class="sql">
CREATE PROCEDURE
person_increment_age (IN id BIGINT,
                      OUT first TEXT,
                      OUT last TEXT,
                      OUT age SMALLINT)
AS $$
  UPDATE person
    SET age = age + 1
    WHERE person.id = id
    RETURNING first, last, age;
$$ LANGUAGE SQL;
  </pre>

  <pre class="cxx">
#pragma db view query("CALL person_increment_age((?))")
struct person_increment_age
{
  std::string first;
  std::string last;
  unsigned short age;
};
  </pre>

  <pre class="cxx">
using query = odb::query&lt;person_increment_age>;

person_increment_age r (
  db.query_value&lt;person_increment_age> (
    query::_val (123) + ",NULL,NULL,NULL"));
  </pre>

  <p>Note that in the <code>CALL</code> statement, output parameters must
    be included in the argument list, normally as <code>NULL</code>.</p>

  <p>A stored procedure that doesn't return anything and has no parameters can
     be called with <code>execute()</code> (<a href="#3.12">3.12 Executing
     Native SQL Statements</a>) rather than <code>query()</code>. Such a
     procedure can also control transactions. For example:</p>

  <pre class="sql">
CREATE PROCEDURE
person_increment_all_ages ()
AS $$
BEGIN
  UPDATE person
    SET age = age + 1;
  COMMIT;
END;
$$ LANGUAGE PLPGSQL;
  </pre>

  <pre class="cxx">
db.connection ()->execute ("CALL person_increment_all_ages()");
  </pre>

  <p>For more information on how to call stored procedures and functions with
     the various return approaches refer to the <code>pgsql/stored-proc</code>
     test in the <code>odb-tests</code> package.</p>

  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="20">20 Oracle Database</a></h1>

  <p>To generate support code for the Oracle database you will need
     to pass the "<code>--database&nbsp;oracle</code>"
     (or "<code>-d&nbsp;oracle</code>") option to the ODB compiler.
     Your application will also need to link to the Oracle ODB runtime
     library (<code>libodb-oracle</code>). All Oracle-specific ODB
     classes are defined in the <code>odb::oracle</code> namespace.</p>

  <h2><a name="20.1">20.1 Oracle Type Mapping</a></h2>

  <p>The following table summarizes the default mapping between basic
     C++ value types and Oracle database types. This mapping can be
     customized on the per-type and per-member basis using the ODB
     Pragma Language (<a href="#14">Chapter 14, "ODB Pragma
     Language"</a>).</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>C++ Type</th>
      <th>Oracle Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>bool</code></td>
      <td><code>NUMBER(1)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>char</code></td>
      <td><code>CHAR(1)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>signed char</code></td>
      <td><code>NUMBER(3)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned char</code></td>
      <td><code>NUMBER(3)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>short</code></td>
      <td><code>NUMBER(5)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned short</code></td>
      <td><code>NUMBER(5)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>int</code></td>
      <td><code>NUMBER(10)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned int</code></td>
      <td><code>NUMBER(10)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>long</code></td>
      <td><code>NUMBER(19)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned long</code></td>
      <td><code>NUMBER(20)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>long long</code></td>
      <td><code>NUMBER(19)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned long long</code></td>
      <td><code>NUMBER(20)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>float</code></td>
      <td><code>BINARY_FLOAT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>double</code></td>
      <td><code>BINARY_DOUBLE</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>std::string</code></td>
      <td><code>VARCHAR2(512)</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>char[N]</code></td>
      <td><code>VARCHAR2(N-1)</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>It is possible to map the <code>char</code> C++ type to an integer
     database type (for example, <code>NUMBER(3)</code>) using the
     <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section 14.4.3,
     "<code>type</code>"</a>).</p>

  <p>In Oracle empty <code>VARCHAR2</code> and <code>NVARCHAR2</code>
     strings are represented as a <code>NULL</code> value. As a result,
     columns of the <code>std::string</code> and <code>char[N]</code>
     types are by default declared as <code>NULL</code> except for
     primary key columns. However, you can override this by explicitly
     declaring such columns as <code>NOT NULL</code> with the
     <code>db&nbsp;not_null</code> pragma (<a href="#14.4.6">Section
     14.4.6, "<code>null/not_null</code>"</a>). This also means that for
     object ids that are mapped to these Oracle types, an empty string is
     an invalid value.</p>

  <p>Additionally, by default, C++ enums and C++11 enum classes are
     automatically mapped to the Oracle types corresponding to their
     underlying integral types (see table above). The default
     <code>NULL</code> semantics is <code>NOT NULL</code>. For
     example:</p>

  <pre class="cxx">
enum color {red, green, blue};
enum class taste: unsigned char
{
  bitter = 1,
  sweet,
  sour = 4,
  salty
};

#pragma db object
class object
{
  ...

  color color_; // Automatically mapped to NUMBER(10).
  taste taste_; // Automatically mapped to NUMBER(3).
};
  </pre>

  <p>It is also possible to add support for additional Oracle types,
     such as <code>XML</code>, geospatial types, user-defined types,
     and collections (arrays, table types). For more information, refer to
     <a href="#14.8">Section 14.8, "Database Type Mapping
     Pragmas"</a>.</p>

  <h3><a name="20.1.1">20.1.1 String Type Mapping</a></h3>

  <p>The Oracle ODB runtime library provides support for mapping the
     <code>std::string</code>, <code>char[N]</code>, and
     <code>std::array&lt;char, N></code> types to the Oracle <code>CHAR</code>,
     <code>VARCHAR2</code>, <code>CLOB</code>, <code>NCHAR</code>,
     <code>NVARCHAR2</code>, and <code>NCLOB</code> types. However,
     these mappings are not enabled by default (in particular, by
     default, <code>std::array</code> will be treated as a container).
     To enable the alternative mappings for these types we need to
     specify the database type explicitly using the <code>db&nbsp;type</code>
     pragma (<a href="#14.4.3">Section 14.4.3, "<code>type</code>"</a>),
     for example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  #pragma db type ("CHAR(2)")
  char state_[2];

  #pragma db type ("VARCHAR(128)") null
  std::string name_;

  #pragma db type ("CLOB")
  std::string text_;
};
  </pre>

  <p>Alternatively, this can be done on the per-type basis, for example:</p>

  <pre class="cxx">
#pragma db value(std::string) type("VARCHAR(128)") null

#pragma db object
class object
{
  ...

  std::string name_; // Mapped to VARCHAR(128).

  #pragma db type ("CLOB")
  std::string text_; // Mapped to CLOB.
};
  </pre>

  <p>The <code>char[N]</code> and <code>std::array&lt;char, N></code> values
     may or may not be zero-terminated. When extracting such values from the
     database, ODB will append the zero terminator if there is enough
     space.</p>

  <h3><a name="20.1.2">20.1.2 Binary Type Mapping</a></h3>

  <p>The Oracle ODB runtime library provides support for mapping the
     <code>std::vector&lt;char></code>,
     <code>std::vector&lt;unsigned&nbsp;char></code>,
     <code>char[N]</code>, <code>unsigned&nbsp;char[N]</code>,
     <code>std::array&lt;char, N></code>, and
     <code>std::array&lt;unsigned char, N></code>
     types to the Oracle <code>BLOB</code> and <code>RAW</code> types.
     However, these mappings are not enabled by default (in particular, by
     default, <code>std::vector</code> and <code>std::array</code> will be
     treated as containers). To enable the alternative mappings for these
     types we need to specify the database type explicitly using the
     <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section 14.4.3,
     "<code>type</code>"</a>), for example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  #pragma db type("BLOB")
  std::vector&lt;char> buf_;

  #pragma db type("RAW(16)")
  unsigned char uuid_[16];
};
  </pre>

  <p>Alternatively, this can be done on the per-type basis, for example:</p>

  <pre class="cxx">
using buffer = std::vector&lt;char>;
#pragma db value(buffer) type("BLOB")

#pragma db object
class object
{
  ...

  buffer buf_; // Mapped to BLOB.
};
  </pre>

  <p>Note also that in native queries (<a href="#4">Chapter 4, "Querying
     the Database"</a>) <code>char[N]</code> and
     <code>std::array&lt;char, N></code> parameters are by default passed
     as a string rather than a binary. To pass such parameters as a binary,
     we need to specify the database type explicitly in the
     <code>_val()</code>/<code>_ref()</code> calls. Note also that we
     don't need to do this for the integrated queries, for example:</p>

  <pre class="cxx">
char u[16] = {...};

db.query&lt;object> ("uuid = " + query::_val&lt;odb::oracle::id_raw> (u));
db.query&lt;object> (query::uuid == query::_ref (u));
  </pre>

  <h2><a name="20.2">20.2 Oracle Database Class</a></h2>

  <p>The Oracle <code>database</code> class encapsulates the OCI environment
     handle as well as the database connection string and user credentials
     that are used to establish connections to the database. It has the
     following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace oracle
  {
    class database: public odb::database
    {
    public:
      database (const std::string&amp; user,
                const std::string&amp; password,
                const std::string&amp; db,
                ub2 charset = 0,
                ub2 ncharset = 0,
                OCIEnv* environment = 0,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      database (const std::string&amp; user,
                const std::string&amp; password,
                const std::string&amp; service,
                const std::string&amp; host,
                unsigned int port = 0,
                ub2 charset = 0,
                ub2 ncharset = 0,
                OCIEnv* environment = 0,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      database (int&amp; argc,
                char* argv[],
                bool erase = false,
                ub2 charset = 0,
                ub2 ncharset = 0,
                OCIEnv* environment = 0,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      static void
      print_usage (std::ostream&amp;);

    public:
      const std::string&amp;
      user () const;

      const std::string&amp;
      password () const;

      const std::string&amp;
      db () const;

      const std::string&amp;
      service () const;

      const std::string&amp;
      host () const;

      unsigned int
      port () const;

      ub2
      charset () const;

      ub2
      ncharset () const;

      OCIEnv*
      environment ();

    public:
      connection_ptr
      connection ();
    };
  }
}
  </pre>

  <p>You will need to include the <code>&lt;odb/oracle/database.hxx></code>
     header file to make this class available in your application.</p>

  <p>The overloaded <code>database</code> constructors allow us to specify the
     Oracle database parameters that should be used when connecting to the
     database. The <code>db</code> argument in the first constructor is a
     connection identifier that specifies the database to connect to. For more
     information on the format of the connection identifier, refer to the
     Oracle documentation.</p>

  <p>The second constructor allows us to specify the individual components
     of a connection identifier as the <code>service</code>, <code>host</code>,
     and <code>port</code> arguments. If the <code>host</code> argument is
     empty, then localhost is used by default. Similarly, if the
     <code>port</code> argument is zero, then the default port is used.</p>

  <p>The last constructor extracts the database parameters
     from the command line. The following options are recognized:</p>

  <pre class="terminal">
  --user &lt;login>
  --password &lt;password>
  --database &lt;connect-id>
  --service &lt;name>
  --host &lt;host>
  --port &lt;integer>
  --options-file &lt;file>
  </pre>

  <p>The <code>--options-file</code> option allows us to specify some
     or all of the database options in a file with each option appearing
     on a separate line followed by a space and an option value. Note that it
     is invalid to specify the <code>--database</code> option
     together with <code>--service</code>, <code>--host</code>, or
     <code>--port</code> options.</p>

  <p>If the <code>erase</code> argument to this constructor is true,
     then the above options are removed from the <code>argv</code>
     array and the <code>argc</code> count is updated accordingly.
     This is primarily useful if your application accepts other
     options or arguments and you would like to get the Oracle
     options out of the <code>argv</code> array.</p>

  <p>This constructor throws the <code>odb::oracle::cli_exception</code>
     exception if the Oracle option values are missing or invalid. See section
     <a href="#20.4">Section 20.4, "Oracle Exceptions"</a> for more
     information on this exception.</p>

  <p>The static <code>print_usage()</code> function prints the list of options
     with short descriptions that are recognized by this constructor.</p>

  <p>Additionally, all the constructors have the <code>charset</code>,
     <code>ncharset</code>, and <code>environment</code> arguments.
     The <code>charset</code> argument specifies the client-side database
     character encoding. Character data corresponding to the <code>CHAR</code>,
     <code>VARCHAR2</code>, and <code>CLOB</code> types will be delivered
     to and received from the application in this encoding. Similarly,
     the <code>ncharset</code> argument specifies the client-side national
     character encoding. Character data corresponding to the <code>NCHAR</code>,
     <code>NVARCHAR2</code>, and <code>NCLOB</code> types will be delivered
     to and received from the application in this encoding. For the complete
     list of available character encoding values, refer to the Oracle
     documentation. Commonly used encoding values are <code>873</code>
     (UTF-8), <code>31</code> (ISO-8859-1), and <code>1000</code> (UTF-16).
     If the database character encoding is not specified, then the
     <code>NLS_LANG</code> environment/registry variable is used. Similarly,
     if the national character encoding is not specified, then the
     <code>NLS_NCHAR</code> environment/registry variable is used. For more
     information on character encodings, refer to the
     <code>OCIEnvNlsCreate()</code> function in the Oracle Call Interface
     (OCI) documentation.</p>

  <p>The <code>environment</code> argument allows us to provide a custom
     OCI environment handle. If this argument is not <code>NULL</code>,
     then the passed handle is used in all the OCI function calls made
     by this <code>database</code> class instance. Note also that the
     <code>database</code> instance does not assume ownership of the
     passed environment handle and this handle should be valid for
     the lifetime of the <code>database</code> instance. If a custom
     environment handle is used, then the <code>charset</code> and
     <code>ncharset</code> arguments have no effect.</p>

  <p>The last argument to all of the constructors is a pointer to the
     connection factory. In C++98/03, it is <code>std::auto_ptr</code> while
     in C++11 <code>std::unique_ptr</code> is used instead. If we pass a
     non-<code>NULL</code> value, the database instance assumes ownership
     of the factory instance. The connection factory interface as well as
     the available implementations are described in the next section.</p>

  <p>The set of accessor functions following the constructors allows us
     to query the parameters of the <code>database</code> instance.</p>

  <p>The <code>connection()</code> function returns a pointer to the
     Oracle database connection encapsulated by the
     <code>odb::oracle::connection</code> class. For more information
     on <code>oracle::connection</code>, refer to <a href="#20.3">Section
     20.3, "Oracle Connection and Connection Factory"</a>.</p>

  <h2><a name="20.3">20.3 Oracle Connection and Connection Factory</a></h2>

  <p>The <code>oracle::connection</code> class has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace oracle
  {
    class connection: public odb::connection
    {
    public:
      connection (database&amp;);
      connection (database&amp;, OCISvcCtx*);

      OCISvcCtx*
      handle ();

      OCIError*
      error_handle ();

      details::buffer&amp;
      lob_buffer ();
    };

    using connection_ptr = details::shared_ptr&lt;connection>;
  }
}
  </pre>

  <p>For more information on the <code>odb::connection</code> interface, refer
     to <a href="#3.6">Section 3.6, "Connections"</a>. The first overloaded
     <code>oracle::connection</code> constructor creates a new OCI service
     context. The OCI statement caching is enabled for the underlying session
     while the OCI connection pooling and session pooling are not used. The
     second constructor allows us to create a <code>connection</code> instance by
     providing an already connected Oracle service context. Note that the
     <code>connection</code> instance assumes ownership of this handle. The
     <code>handle()</code> accessor returns the OCI service context handle
     associated with the <code>connection</code> instance.</p>

  <p>An OCI error handle is allocated for each <code>connection</code>
     instance and is available via the <code>error_handle()</code> accessor
     function.</p>

  <p>Additionally, each <code>connection</code> instance maintains a large
     object (LOB) buffer. This buffer is used by the Oracle ODB runtime
     as an intermediate storage for piecewise handling of LOB data.
     By default, the LOB buffer has zero initial capacity and is
     expanded to 4096 bytes when the first LOB operation is performed.
     If your application requires a bigger or smaller LOB buffer, you can
     specify a custom capacity using the <code>lob_buffer()</code>
     accessor.</p>

  <p>The <code>oracle::connection_factory</code> abstract class has the
     following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace oracle
  {
    class connection_factory
    {
    public:
      virtual void
      database (database&amp;) = 0;

      virtual connection_ptr
      connect () = 0;
    };
  }
}
  </pre>

  <p>The <code>database()</code> function is called when a connection
     factory is associated with a database instance. This happens in
     the <code>odb::oracle::database</code> class constructors. The
     <code>connect()</code> function is called whenever a database
     connection is requested.</p>

  <p>The two implementations of the <code>connection_factory</code>
     interface provided by the Oracle ODB runtime are
     <code>new_connection_factory</code> and
     <code>connection_pool_factory</code>. You will need to include
     the <code>&lt;odb/oracle/connection-factory.hxx></code>
     header file to make the <code>connection_factory</code> interface
     and these implementation classes available in your application.</p>

  <p>The <code>new_connection_factory</code> class creates a new
     connection whenever one is requested. When a connection is no
     longer needed, it is released and closed. The
     <code>new_connection_factory</code> class has the following
     interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace oracle
  {
    class new_connection_factory: public connection_factory
    {
    public:
      new_connection_factory ();
    };
};
  </pre>

  <p>The <code>connection_pool_factory</code> class implements a
     connection pool. It has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace oracle
  {
    class connection_pool_factory: public connection_factory
    {
    public:
      connection_pool_factory (std::size_t max_connections = 0,
                               std::size_t min_connections = 0);

    protected:
      class pooled_connection: public connection
      {
      public:
        pooled_connection (database_type&amp;);
        pooled_connection (database_type&amp;, OCISvcCtx*);
      };

      using pooled_connection_ptr = details::shared_ptr&lt;pooled_connection>;

      virtual pooled_connection_ptr
      create ();
    };
};
  </pre>

  <p>The <code>max_connections</code> argument in the
     <code>connection_pool_factory</code> constructor specifies the maximum
     number of concurrent connections that this pool factory will
     maintain. Similarly, the <code>min_connections</code> argument
     specifies the minimum number of available connections that
     should be kept open.</p>

  <p>Whenever a connection is requested, the pool factory first
     checks if there is an unused connection that can be returned.
     If there is none, the pool factory checks the
     <code>max_connections</code> value to see if a new connection
     can be created. If the total number of connections maintained
     by the pool is less than this value, then a new connection is
     created and returned. Otherwise, the caller is blocked until
     a connection becomes available.</p>

  <p>When a connection is released, the pool factory first checks
     if there are blocked callers waiting for a connection. If so, then
     one of them is unblocked and is given the connection. Otherwise,
     the pool factory checks whether the total number of connections
     maintained by the pool is greater than the <code>min_connections</code>
     value. If that's the case, the connection is closed. Otherwise, the
     connection is added to the pool of available connections to be
     returned on the next request. In other words, if the number of
     connections maintained by the pool exceeds <code>min_connections</code>
     and there are no callers waiting for a new connection,
     the pool will close the excess connections.</p>

  <p>If the <code>max_connections</code> value is 0, then the pool will
     create a new connection whenever all of the existing connections
     are in use. If the <code>min_connections</code> value is 0, then
     the pool will never close a connection and instead maintain all
     the connections that were ever created.</p>

  <p>The <code>create()</code> virtual function is called whenever the
     pool needs to create a new connection. By deriving from the
     <code>connection_pool_factory</code> class and overriding this
     function we can implement custom connection establishment
     and configuration.</p>

  <p>If you pass <code>NULL</code> as the connection factory to one of the
     <code>database</code> constructors, then the
     <code>connection_pool_factory</code> instance will be created by default
     with the min and max connections values set to <code>0</code>. The
     following code fragment shows how we can pass our own connection factory
     instance:</p>

  <pre class="cxx">
#include &lt;odb/database.hxx>

#include &lt;odb/oracle/database.hxx>
#include &lt;odb/oracle/connection-factory.hxx>

int
main (int argc, char* argv[])
{
  unique_ptr&lt;odb::oracle::connection_factory> f (
    new odb::oracle::connection_pool_factory (20));

  unique_ptr&lt;odb::database> db (
    new oracle::database (argc, argv, false, 0, 0, 0, f));
}
  </pre>

  <h2><a name="20.4">20.4 Oracle Exceptions</a></h2>

  <p>The Oracle ODB runtime library defines the following
     Oracle-specific exceptions:</p>

  <pre class="cxx">
namespace odb
{
  namespace oracle
  {
    class database_exception: odb::database_exception
    {
    public:
      class record
      {
      public:
        sb4
        error () const;

        const std::string&amp;
        message () const;
      };

      using records = std::vector&lt;record>;

      using size_type = records::size_type;
      using iterator = records::const_iterator;

      iterator
      begin () const;

      iterator
      end () const;

      size_type
      size () const;

      virtual const char*
      what () const throw ();
    };

    class cli_exception: odb::exception
    {
    public:
      virtual const char*
      what () const throw ();
    };

    class invalid_oci_handle: odb::exception
    {
    public:
      virtual const char*
      what () const throw ();
    };
  }
}
  </pre>

  <p>You will need to include the <code>&lt;odb/oracle/exceptions.hxx></code>
     header file to make these exceptions available in your application.</p>

  <p>The <code>odb::oracle::database_exception</code> is thrown if
     an Oracle database operation fails. The Oracle-specific error
     information is stored as a series of records, each containing
     the error code as a signed 4-byte integer and the message string.
     All this information is also combined and returned in a
     human-readable form by the <code>what()</code> function.</p>

  <p>The <code>odb::oracle::cli_exception</code> is thrown by the
     command line parsing constructor of the <code>odb::oracle::database</code>
     class if the Oracle option values are missing or invalid. The
     <code>what()</code> function returns a human-readable description
     of an error.</p>

  <p>The <code>odb::oracle::invalid_oci_handle</code> is thrown if an
     invalid handle is passed to an OCI function or if an OCI function
     was unable to allocate a handle. The former normally indicates
     a programming error while the latter indicates an out of memory
     condition. The <code>what()</code> function returns a human-readable
     description of an error.</p>

  <h2><a name="20.5">20.5 Oracle Limitations</a></h2>

  <p>The following sections describe Oracle-specific limitations imposed
     by the current Oracle and ODB runtime versions.</p>

  <h3><a name="20.5.1">20.5.1 Identifier Truncation</a></h3>

  <p>Oracle limits the length of database identifiers (table, column, etc.,
     names) to 30 characters. The ODB compiler automatically truncates
     any identifier that is longer than 30 characters. This, however,
     can lead to duplicate names. A common symptom of this problem
     are errors during the database schema creation indicating
     that a database object with the same name already exists. To
     resolve this problem we can assign custom, shorter identifiers
     using the <code>db&nbsp;table</code> and <code>db&nbsp;column</code>
     pragmas (<a href="#14">Chapter 14, "ODB Pragma Language")</a>. For
     example:</p>

  <pre class="cxx">
#pragma db object
class long_class_name
{
  ...

  std::vector&lt;int> long_container_x_;
  std::vector&lt;int> long_container_y_;
};
  </pre>

  <p>In the above example, the names of the two container tables will be
     <code>long_class_name_long_container_x_</code> and
     <code>long_class_name_long_container_y_</code>. However, when
     truncated to 30 characters, they both become
     <code>long_class_name_long_container</code>. To resolve this
     collision we can assign a custom table name for each container:</p>

  <pre class="cxx">
#pragma db object
class long_class_name
{
  ...

  #pragma db table("long_class_name_cont_x")
  std::vector&lt;int> long_container_x_;

  #pragma db table("long_class_name_cont_y")
  std::vector&lt;int> long_container_y_;
};
  </pre>

  <h3><a name="20.5.2">20.5.2 Query Result Caching</a></h3>

  <p>Oracle ODB runtime implementation does not perform query result caching
     (<a href="#4.4">Section 4.4, "Query Result"</a>) even when explicitly
     requested. The OCI API supports interleaving execution of multiple
     prepared statements on a single connection. As a result, with OCI,
     it is possible to have multiple uncached results and calls to other
     database functions do not invalidate them. The only limitation of
     the uncached Oracle results is the unavailability of the
     <code>result::size()</code> function. If you call this function on
     an Oracle query result, then the <code>odb::result_not_cached</code>
     exception (<a href="#3.14">Section 3.14, "ODB Exceptions"</a>) is
     always thrown. Future versions of the Oracle ODB runtime library
     may add support for result caching.</p>

  <h3><a name="20.5.3">20.5.3 Foreign Key Constraints</a></h3>

  <p>ODB assumes the standard SQL behavior which requires that
     foreign key constraints checking is deferred until the
     transaction is committed. Default Oracle behavior is
     to check such constraints immediately. As a result, when
     used with ODB, a custom database schema that defines foreign
     key constraints may need to declare such constraints as
     <code>INITIALLY DEFERRED</code>, as shown in the following example.
     By default, schemas generated by the ODB compiler meet this requirement
     automatically.</p>

  <pre class="sql">
CREATE TABLE Employee (
  ...
  employer NUMBER(20) REFERENCES Employer(id)
           DEFERRABLE INITIALLY DEFERRED);
  </pre>

  <p>You can override the default behavior and instruct the ODB
     compiler to generate non-deferrable foreign keys by specifying
     the <code>--fkeys-deferrable-mode not_deferrable</code> ODB
     compiler option. Note, however, that in this case the order in
     which you persist, update, and erase objects within a transaction
     becomes important.</p>

  <h3><a name="20.5.4">20.5.4 Unique Constraint Violations</a></h3>

  <p>Due to the granularity of the Oracle error codes, it is impossible
     to distinguish between the duplicate primary key and other unique
     constraint violations. As a result, when making an object persistent,
     the Oracle ODB runtime will translate all unique constraint violation
     errors to the <code>object_already_persistent</code> exception
     (<a href="#3.14">Section 3.14, "ODB Exceptions"</a>).</p>

  <h3><a name="20.5.5">20.5.5 Large <code>FLOAT</code> and
      <code>NUMBER</code> Types</a></h3>

  <p>The Oracle <code>FLOAT</code> type with a binary precision greater
     than 53 and fixed-point <code>NUMBER</code> type with a decimal
     precision greater than 15 cannot be automatically extracted
     into the C++ <code>float</code> and <code>double</code> types.
     Instead, the Oracle ODB runtime uses a 21-byte buffer containing
     the binary representation of a value as an image type for such
     <code>FLOAT</code> and <code>NUMBER</code> types. In order to
     convert them into an application-specific large number representation,
     you will need to provide a suitable <code>value_traits</code>
     template specialization. For more information on the binary format
     used to store the <code>FLOAT</code> and <code>NUMBER</code> values,
     refer to the Oracle Call Interface (OCI) documentation.</p>

  <p>An alternative approach to accessing large <code>FLOAT</code> and
     <code>NUMBER</code> values is to map these type to one of the
     natively supported ones, as discussed in <a href="#14.8">Section
     14.8, "Database Type Mapping Pragmas"</a>.</p>

  <p>Note that a <code>NUMBER</code> type that is used to represent a
     floating point number (declared by specifying <code>NUMBER</code>
     without any range and scale) can be extracted into the C++
     <code>float</code> and <code>double</code> types.</p>

  <h3><a name="20.5.6">20.5.6 Timezones</a></h3>

  <p>ODB does not currently support the Oracle date-time types with timezone
     information. However, these types can be accessed by mapping them to
     one of the natively supported types, as discussed in
     <a href="#14.8">Section 14.8, "Database Type Mapping Pragmas"</a>.</p>

  <h3><a name="20.5.7">20.5.7 <code>LONG</code> Types</a></h3>

  <p>ODB does not support the deprecated Oracle <code>LONG</code> and
     <code>LONG RAW</code> data types. However, these types can be accessed
     by mapping them to one of the natively supported types, as discussed
     in <a href="#14.8">Section 14.8, "Database Type Mapping Pragmas"</a>.</p>

  <h3><a name="20.5.8">20.5.8 LOB Types and By-Value Accessors/Modifiers</a></h3>

  <p>As discussed in <a href="#14.4.5">Section 14.4.5,
     "<code>get</code>/<code>set</code>/<code>access</code>"</a>, by-value
     accessor and modifier expressions cannot be used with data members
     of Oracle large object (LOB) data types: <code>BLOB</code>,
     <code>CLOB</code>, and <code>NCLOB</code>. The Oracle ODB runtime
     uses streaming for reading/writing LOB data directly from/to
     data members. As a result, by-reference accessors and modifiers
     should be used for these data types.</p>

  <h3><a name="20.5.9">20.5.9 Database Schema Evolution</a></h3>

  <p>In Oracle, the type of the <code>name</code> column in the
     <code>schema_version</code> table is <code>VARCHAR2(512)</code>.
     Because this column is a primary key and <code>VARCHAR2</code>
     represents empty strings as <code>NULL</code> values, it is
     impossible to store an empty string in this column, which
     is what is used to represent the default schema name. As a
     result, in Oracle, the empty schema name is stored as a
     string containing a single space character. ODB performs
     all the necessary translations automatically and normally
     you do not need to worry about this implementation detail
     unless you are querying or modifying the <code>schema_version</code>
     table directly.</p>

  <h2><a name="20.6">20.6 Oracle Index Definitions</a></h2>

  <p>When the <code>index</code> pragma (<a href="#14.7">Section 14.7,
     "Index Definition Pragmas"</a>) is used to define an Oracle index,
     the <code>type</code> clause specifies the index type (for example,
     <code>UNIQUE</code>, <code>BITMAP</code>), the <code>method</code>
     clause is not used, and the <code>options</code> clause specifies
     additional index properties, such as partitioning, table spaces, etc.
     The column options can be used to specify the sort order. For example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  std::string name_;

  #pragma db index                     \
             type("BITMAP")            \
             member(name_, "DESC")     \
             options("TABLESPACE TBS1")
};
  </pre>

  <p>Index names in Oracle are schema-global. To avoid name clashes,
     ODB automatically prefixes each index name with the table name on
     which it is defined.</p>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="21">21 Microsoft SQL Server Database</a></h1>

  <p>To generate support code for the SQL Server database you will need
     to pass the "<code>--database&nbsp;mssql</code>"
     (or "<code>-d&nbsp;mssql</code>") option to the ODB compiler.
     Your application will also need to link to the SQL Server ODB runtime
     library (<code>libodb-mssql</code>). All SQL Server-specific ODB
     classes are defined in the <code>odb::mssql</code> namespace.</p>

  <h2><a name="21.1">21.1 SQL Server Type Mapping</a></h2>

  <p>The following table summarizes the default mapping between basic
     C++ value types and SQL Server database types. This mapping can be
     customized on the per-type and per-member basis using the ODB
     Pragma Language (<a href="#14">Chapter 14, "ODB Pragma Language"</a>).</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>C++ Type</th>
      <th>SQL Server Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>bool</code></td>
      <td><code>BIT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>char</code></td>
      <td><code>CHAR(1)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>signed char</code></td>
      <td><code>TINYINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned char</code></td>
      <td><code>TINYINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>short</code></td>
      <td><code>SMALLINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned short</code></td>
      <td><code>SMALLINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>int</code></td>
      <td><code>INT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned int</code></td>
      <td><code>INT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>long</code></td>
      <td><code>BIGINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned long</code></td>
      <td><code>BIGINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>long long</code></td>
      <td><code>BIGINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>unsigned long long</code></td>
      <td><code>BIGINT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>float</code></td>
      <td><code>REAL</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>double</code></td>
      <td><code>FLOAT</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>std::string</code></td>
      <td><code>VARCHAR(512)/VARCHAR(256)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>char[N]</code></td>
      <td><code>VARCHAR(N-1)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>std::wstring</code></td>
      <td><code>NVARCHAR(512)/NVARCHAR(256)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>wchar_t[N]</code></td>
      <td><code>NVARCHAR(N-1)</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

    <tr>
      <td><code>GUID</code></td>
      <td><code>UNIQUEIDENTIFIER</code></td>
      <td><code>NOT NULL</code></td>
    </tr>

  </table>

  <p>It is possible to map the <code>char</code> C++ type to an integer
     database type (for example, <code>TINYINT</code>) using the
     <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section 14.4.3,
     "<code>type</code>"</a>).</p>

  <p>Note that the <code>std::string</code> and <code>std::wstring</code>
     types are mapped differently depending on whether a member of one of
     these types is an object id or not. If the member is an object id,
     then for this member <code>std::string</code> is mapped
     to <code>VARCHAR(256)</code> and <code>std::wstring</code> &mdash;
     to <code>NVARCHAR(256)</code>. Otherwise, <code>std::string</code>
     is mapped to <code>VARCHAR(512)</code> and <code>std::wstring</code>
     &mdash; to <code>NVARCHAR(512)</code>. Note also that you can
     always change this mapping using the <code>db&nbsp;type</code> pragma
     (<a href="#14.4.3">Section 14.4.3, "<code>type</code>"</a>).</p>

  <p>Additionally, by default, C++ enums and C++11 enum classes are
     automatically mapped to the SQL Server types corresponding to their
     underlying integral types (see table above). The default
     <code>NULL</code> semantics is <code>NOT NULL</code>. For
     example:</p>

  <pre class="cxx">
enum color {red, green, blue};
enum class taste: unsigned char
{
  bitter = 1,
  sweet,
  sour = 4,
  salty
};

#pragma db object
class object
{
  ...

  color color_; // Automatically mapped to INT.
  taste taste_; // Automatically mapped to TINYINT.
};
  </pre>

  <p>Note also that because SQL Server does not support unsigned integers,
     the <code>unsigned&nbsp;short</code>, <code>unsigned&nbsp;int</code>, and
     <code>unsigned&nbsp;long</code>/<code>unsigned&nbsp;long&nbsp;long</code> C++ types
     are by default mapped to the <code>SMALLINT</code>, <code>INT</code>,
     and <code>BIGINT</code> SQL Server types, respectively. The sign bit
     of the value stored by the database for these types will contain
     the most significant bit of the actual unsigned value being
     persisted. Similarly, because there is no signed version of the
     <code>TINYINT</code> SQL Server type, by default, the
     <code>signed char</code> C++ type is mapped to <code>TINYINT</code>.
     As a result, the most significant bit of the value stored by the
     database for this type will contain the sign bit of the actual
     signed value being persisted.</p>

  <p>It is also possible to add support for additional SQL Server types,
     such as geospatial types, <code>XML</code>, and user-defined types.
     For more information, refer to <a href="#14.8">Section 14.8, "Database
     Type Mapping Pragmas"</a>.</p>

  <h3><a name="21.1.1">21.1.1 String Type Mapping</a></h3>

  <p>The SQL Server ODB runtime library provides support for mapping the
     <code>std::string</code>, <code>char[N]</code>, and
     <code>std::array&lt;char, N></code> types to the SQL Server
     <code>CHAR</code>, <code>VARCHAR</code>, and <code>TEXT</code>
     types as well as the <code>std::wstring</code>, <code>wchar_t[N]</code>,
     and <code>std::array&lt;wchar_t, N></code> types to <code>NCHAR</code>,
     <code>NVARCHAR</code>, and <code>NTEXT</code>. However, these mappings
     are not enabled by default (in particular, by default,
     <code>std::array</code> will be treated as a container). To enable the
     alternative mappings for these types we need to specify the database
     type explicitly using the <code>db&nbsp;type</code> pragma
     (<a href="#14.4.3">Section 14.4.3, "<code>type</code>"</a>), for
     example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  #pragma db type ("CHAR(2)")
  char state_[2];

  #pragma db type ("NVARCHAR(max)")
  std::wstring text_;
};
  </pre>

  <p>Alternatively, this can be done on the per-type basis, for example:</p>

  <pre class="cxx">
#pragma db value(std::wstring) type("NVARCHAR(max)")

#pragma db object
class object
{
  ...

  std::wstring text_; // Mapped to NVARCHAR(max).
};
  </pre>

  <p>The <code>char[N]</code>, <code>std::array&lt;char, N></code>,
     <code>wchar_t[N]</code>, and <code>std::array&lt;wchar_t, N></code>
     values may or may not be zero-terminated. When extracting such values
     from the database, ODB will append the zero terminator if there is
     enough space.</p>

  <p>See also <a href="#21.1.4">Section 21.1.4, "Long String and Binary
     Types"</a> for certain limitations of long string types.</p>

  <h3><a name="21.1.2">21.1.2 Binary Type and <code>UNIQUEIDENTIFIER</code> Mapping</a></h3>

  <p>The SQL Server ODB runtime library also provides support for mapping the
     <code>std::vector&lt;char></code>,
     <code>std::vector&lt;unsigned&nbsp;char></code>,
     <code>char[N]</code>, <code>unsigned&nbsp;char[N]</code>,
     <code>std::array&lt;char, N></code>, and <code>std::array&lt;unsigned char, N></code>
     types to the SQL Server <code>BINARY</code>, <code>VARBINARY</code>, and
     <code>IMAGE</code> types. There is also support for mapping the
     <code>char[16]</code> array to the SQL Server <code>UNIQUEIDENTIFIER</code>
     type. However, these mappings are not enabled by default (in particular,
     by default, <code>std::vector</code> and <code>std::array</code> will
     be treated as containers). To enable the alternative mappings for these
     types we need to specify the database type explicitly using the
     <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section 14.4.3,
     "<code>type</code>"</a>), for example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  #pragma db type("UNIQUEIDENTIFIER")
  char uuid_[16];

  #pragma db type("VARBINARY(max)")
  std::vector&lt;char> buf_;

  #pragma db type("BINARY(256)")
  unsigned char data_[256];
};
  </pre>

  <p>Alternatively, this can be done on the per-type basis, for example:</p>

  <pre class="cxx">
using buffer = std::vector&lt;char>;
#pragma db value(buffer) type("VARBINARY(max)")

#pragma db object
class object
{
  ...

  buffer buf_; // Mapped to VARBINARY(max).
};
  </pre>

  <p>Note also that in native queries (<a href="#4">Chapter 4, "Querying
     the Database"</a>) <code>char[N]</code> and
     <code>std::array&lt;char, N></code> parameters are by default passed
     as a string rather than a binary. To pass such parameters as a binary,
     we need to specify the database type explicitly in the
     <code>_val()</code>/<code>_ref()</code> calls. Note also that we
     don't need to do this for the integrated queries, for example:</p>

  <pre class="cxx">
char u[16] = {...};

db.query&lt;object> ("uuid = " + query::_val&lt;odb::mssql::id_binary> (u));
db.query&lt;object> (
  "uuid = " + query::_val&lt;odb::mssql::id_uniqueidentifier> (u));
db.query&lt;object> (query::uuid == query::_ref (u));
  </pre>

  <p>See also <a href="#21.1.4">Section 21.1.4, "Long String and Binary
     Types"</a> for certain limitations of long binary types.</p>

  <h3><a name="21.1.3">21.1.3 <code>ROWVERSION</code> Mapping</a></h3>

  <p><code>ROWVERSION</code> is a special SQL Server data type that is
     automatically incremented by the database server whenever a row
     is inserted or updated. As such, it is normally used to implement
     optimistic concurrency and ODB provides support for using
     <code>ROWVERSION</code> instead of the more portable approach
     for optimistic concurrency (<a href="#12">Chapter 12, "Optimistic
     Concurrency"</a>).</p>

  <p><code>ROWVERSION</code> is a 64-bit value which is mapped by ODB
     to <code>unsigned long long</code>. As a result, to use
     <code>ROWVERSION</code> for optimistic concurrency we need to
     make sure that the version column is of the <code>unsigned long
     long</code> type. We also need to explicitly specify that it
     should be mapped to the <code>ROWVERSION</code> data type. For
     example:</p>

  <pre class="cxx">
#pragma db object optimistic
class person
{
  ...

  #pragma db version type("ROWVERSION")
  unsigned long long version_;
};
  </pre>

  <h3><a name="21.1.4">21.1.4 Long String and Binary Types</a></h3>

  <p>For SQL Server, ODB handles character, national character, and
     binary data in two different ways depending on its maximum length.
     If the maximum length (in bytes) is less than or equal to the limit
     specified with the <code>--mssql-short-limit</code> ODB compiler
     option (1024 by default), then it is treated as <i>short data</i>,
     otherwise &mdash; <i>long data</i>. For short data ODB pre-allocates
     an intermediate buffer of the maximum size and binds it directly
     to a parameter or result column. This way the underlying database
     API (ODBC) can read/write directly from/to this buffer. In the case
     of long data, the data is read/written in chunks using the
     <code>SQLGetData()</code>/<code>SQLPutData()</code> ODBC functions.
     While the long data approach reduces the amount of memory used by
     the application, it may require greater CPU resources.</p>

  <p>Long data has a number of limitations. In particular, when setting
     a custom short data limit, make sure that it is sufficiently large
     so that no object id in the application is treated as long data.
     It is also impossible to load an object or view with long data more
     than once as part of a query result iteration (<a href="#4.4">Section
     4.4, "Query Result"</a>). Any such attempt will result in the
     <code>odb::mssql::long_data_reload</code> exception
     (<a href="#21.4">Section 21.4, "SQL Server Exceptions"</a>). For
     example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  int num_;

  #pragma db type("VARCHAR(max)") // Long data.
  std::string str_;
};

using query = odb::query&lt;object>;
using result = odb::result&lt;object>;

transaction t (db.begin ());

result r (db.query&lt;object> (query::num &lt; 100));

for (result::iterator i (r.begin ()); i != r.end (); ++i)
{
  if (!i->str_.empty ()) // First load.
  {
    object o;
    i.load (o); // Error: second load, long_data_reload is thrown.
  }
}

t.commit ();
  </pre>

  <p>Finally, if a native view (<a href="#10.6">Section 10.6, "Native
     Views"</a>) contains one or more long data members, then such
     members should come last both in the select-list of the native
     SQL query and the list of data members in the C++ class.</p>

  <h2><a name="21.2">21.2 SQL Server Database Class</a></h2>

  <p>The SQL Server <code>database</code> class encapsulates the ODBC
     environment handle as well as the server instance address and
     user credentials that are used to establish connections to the
     database. It has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace mssql
  {
    enum protocol
    {
      protocol_auto,
      protocol_tcp, // TCP/IP.
      protocol_lpc, // Shared memory (local procedure call).
      protocol_np   // Named pipes.
    };

    enum transaction_isolation
    {
      isolation_read_uncommitted,
      isolation_read_committed,   // SQL Server default.
      isolation_repeatable_read,
      isolation_snapshot,
      isolation_serializable
    };

    class database: public odb::database
    {
    public:
      using protocol_type = protocol;
      using transaction_isolation_type = transaction_isolation;

      database (const std::string&amp; user,
                const std::string&amp; password,
                const std::string&amp; db,
                const std::string&amp; server,
                const std::string&amp; driver = "",
                const std::string&amp; extra_connect_string = "",
                transaction_isolation_type = isolation_read_committed,
                SQLHENV environment = 0,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      database (const std::string&amp; user,
                const std::string&amp; password,
                const std::string&amp; db,
                protocol_type protocol = protocol_auto,
                const std::string&amp; host = "",
                const std::string&amp; instance = "",
                const std::string&amp; driver = "",
                const std::string&amp; extra_connect_string = "",
                transaction_isolation_type = isolation_read_committed,
                SQLHENV environment = 0,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      database (const std::string&amp; user,
                const std::string&amp; password,
                const std::string&amp; db,
                const std::string&amp; host,
                unsigned int port,
                const std::string&amp; driver = "",
                const std::string&amp; extra_connect_string = "",
                transaction_isolation_type = isolation_read_committed,
                SQLHENV environment = 0,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      database (const std::string&amp; connect_string,
                transaction_isolation_type = isolation_read_committed,
                SQLHENV environment = 0,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      database (int&amp; argc,
                char* argv[],
                bool erase = false,
                const std::string&amp; extra_connect_string = "",
                transaction_isolation_type = isolation_read_committed,
                SQLHENV environment = 0,
                std::[auto|unique]_ptr&lt;connection_factory> = 0);

      static void
      print_usage (std::ostream&amp;);

    public:
      const std::string&amp;
      user () const;

      const std::string&amp;
      password () const;

      const std::string&amp;
      db () const;

      protocol_type
      protocol () const;

      const std::string&amp;
      host () const;

      const std::string&amp;
      instance () const;

      unsigned int
      port () const;

      const std::string&amp;
      server () const;

      const std::string&amp;
      driver () const;

      const std::string&amp;
      extra_connect_string () const;

      transaction_isolation_type
      transaction_isolation () const;

      const std::string&amp;
      connect_string () const;

      SQLHENV
      environment ();

    public:
      connection_ptr
      connection ();
    };
  }
}
  </pre>

  <p>You will need to include the <code>&lt;odb/mssql/database.hxx></code>
     header file to make this class available in your application.</p>

  <p>The overloaded <code>database</code> constructors allow us to specify the
     SQL Server database parameters that should be used when connecting to the
     database. The <code>user</code> and <code>password</code> arguments
     specify the login name and password. If <code>user</code> is empty,
     then Windows authentication is used and the <code>password</code>
     argument is ignored. The <code>db</code> argument specifies the
     database name to open. If it is empty, then the default database for
     the user is used.</p>

  <p>The <code>server</code> argument in the first constructor specifies
     the SQL Server instance address in the standard SQL Server address
     format:</p>

  <p>
  <code>[<i>protocol</i><b>:</b>]<i>host</i>[<b>\</b><i>instance</i>][<b>,</b><i>port</i>]</code>
  </p>

  <p>Where <code><i>protocol</i></code> can be <code>tcp</code>
    (TCP/IP), <code>lpc</code> (shared memory), or
    <code>np</code> (named pipe). If protocol is not specified, then a
    suitable protocol is automatically selected based on the SQL Server
    Native Client configuration. The <code><i>host</i></code> component
    can be a host name or an IP address. If <code><i>instance</i></code>
    is not specified, then the default SQL Server instance is assumed.
    If port is not specified, then the default SQL Server port is
    used (1433). Note that you would normally specify either the
    instance name or the port, but not both. If both are specified,
    then the instance name is ignored by the SQL Server Native Client
    ODBC driver. For more information on the format of the SQL
    Server address, refer to the SQL Server Native Client ODBC
    driver documentation.</p>

  <p>The second and third constructors allow us to specify all these address
     components (protocol, host, instance, and port) as separate
     arguments. The third constructor always connects using TCP/IP
     to the specified host and port.</p>

  <p>The <code>driver</code> argument specifies the SQL Server Native
     Client ODBC driver that should be used to connect to the database.
     If not specified, then the latest available version is used. The
     following examples show common ways of connecting to the database
     using the first three constructors:</p>

  <pre class="cxx">
// Connect to the default SQL Server instance on the local machine
// using the default protocol. Login as 'test' with password 'secret'
// and open the 'example_db' database.
//
odb::mssql::database db1 ("test",
                          "secret",
                          "example_db");

// As above except use Windows authentication and open the default
// database for this user.
//
odb::mssql::database db2 ("",
                          "",
                          "");

// Connect to the default SQL Server instance on 'onega' using the
// default protocol. Login as 'test' with password 'secret' and open
// the 'example_db' database.
//
odb::mssql::database db3 ("test",
                          "secret",
                          "example_db"
                          "onega");

// As above but connect to the 'production' SQL Server instance.
//
odb::mssql::database db4 ("test",
                          "secret",
                          "example_db"
                          "onega\\production");

// Same as above but specify protocol, host, and instance as separate
// arguments.
//
odb::mssql::database db5 ("test",
                          "secret",
                          "example_db",
                          odb::mssql::protocol_auto,
                          "onega",
                          "production");

// As above, but use TCP/IP as the protocol.
//
odb::mssql::database db6 ("test",
                          "secret",
                          "example_db"
                          "tcp:onega\\production");

// Same as above but using separate arguments.
//
odb::mssql::database db7 ("test",
                          "secret",
                          "example_db",
                          odb::mssql::protocol_tcp,
                          "onega",
                          "production");

// As above, but use TCP/IP port instead of the instance name.
//
odb::mssql::database db8 ("test",
                          "secret",
                          "example_db"
                          "tcp:onega,1435");

// Same as above but using separate arguments. Note that here we
// don't need to specify protocol explicitly since it can only
// be TCP/IP.
//
odb::mssql::database db9 ("test",
                          "secret",
                          "example_db",
                          "onega",
                          1435);

// As above but use the specific SQL Server Native Client ODBC
// driver version.
//
odb::mssql::database dbA ("test",
                          "secret",
                          "example_db"
                          "tcp:onega,1435",
                          "SQL Server Native Client 10.0");
  </pre>


  <p>The fourth constructor allows us to pass a custom ODBC connection
     string that provides all the information necessary to connect to
     the database. Note also that all the other constructors have the
     <code>extra_connect_string</code> argument which can be used to
     specify additional ODBC connection attributes. For more information
     on the format of the ODBC connection string, refer to the SQL
     Server Native Client ODBC driver documentation.</p>

  <p>The last constructor extracts the database parameters
     from the command line. The following options are recognized:</p>

  <pre class="terminal">
  --user | -U &lt;login>
  --password | -P &lt;password>
  --database | -d &lt;name>
  --server | -S &lt;address>
  --driver &lt;name>
  --options-file &lt;file>
  </pre>

  <p>The <code>--options-file</code> option allows us to specify some
     or all of the database options in a file with each option appearing
     on a separate line followed by a space and an option value.</p>

  <p>If the <code>erase</code> argument to this constructor is true,
     then the above options are removed from the <code>argv</code>
     array and the <code>argc</code> count is updated accordingly.
     This is primarily useful if your application accepts other
     options or arguments and you would like to get the SQL Server
     options out of the <code>argv</code> array.</p>

  <p>This constructor throws the <code>odb::mssql::cli_exception</code>
     exception if the SQL Server option values are missing or invalid. See
     section <a href="#21.4">Section 21.4, "SQL Server Exceptions"</a> for
     more information on this exception.</p>

  <p>The static <code>print_usage()</code> function prints the list of options
     with short descriptions that are recognized by this constructor.</p>

  <p>Additionally, all the constructors have the <code>transaction_isolation</code>
     and <code>environment</code> arguments. The <code>transaction_isolation</code>
     argument allows us to specify an alternative transaction isolation level
     that should be used by all the connections created by this database instance.
     The <code>environment</code> argument allows us to provide a custom ODBC
     environment handle. If this argument is not <code>NULL</code>, then the
     passed handle is used in all the ODBC function calls made by this
     <code>database</code> instance. Note also that the <code>database</code>
     instance does not assume ownership of the passed environment handle and
     this handle should be valid for the lifetime of the <code>database</code>
     instance.</p>

  <p>The last argument to all of the constructors is a pointer to the
     connection factory. In C++98/03, it is <code>std::auto_ptr</code> while
     in C++11 <code>std::unique_ptr</code> is used instead. If we pass a
     non-<code>NULL</code> value, the database instance assumes ownership
     of the factory instance. The connection factory interface as well as
     the available implementations are described in the next section.</p>

  <p>The set of accessor functions following the constructors allows us
     to query the parameters of the <code>database</code> instance.</p>

  <p>The <code>connection()</code> function returns a pointer to the
     SQL Server database connection encapsulated by the
     <code>odb::mssql::connection</code> class. For more information
     on <code>mssql::connection</code>, refer to <a href="#21.3">Section
     21.3, "SQL Server Connection and Connection Factory"</a>.</p>

  <h2><a name="21.3">21.3 SQL Server Connection and Connection Factory</a></h2>

  <p>The <code>mssql::connection</code> class has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace mssql
  {
    class connection: public odb::connection
    {
    public:
      connection (database&amp;);
      connection (database&amp;, SQLHDBC handle);

      SQLHDBC
      handle ();

      details::buffer&amp;
      long_data_buffer ();
    };

    using connection_ptr = details::shared_ptr&lt;connection>;
  }
}
  </pre>

  <p>For more information on the <code>odb::connection</code> interface, refer
     to <a href="#3.6">Section 3.6, "Connections"</a>. The first overloaded
     <code>mssql::connection</code> constructor creates a new ODBC connection.
     The created connection is configured to use the manual commit mode with
     multiple active result sets (MARS) enabled. The second constructor allows
     us to create a <code>connection</code> instance by providing an already
     established ODBC connection. Note that the <code>connection</code>
     instance assumes ownership of this handle. The <code>handle()</code>
     accessor returns the underlying ODBC connection handle associated with
     the <code>connection</code> instance.</p>

  <p>Additionally, each <code>connection</code> instance maintains a long
     data buffer. This buffer is used by the SQL Server ODB runtime
     as an intermediate storage for piecewise handling of long data.
     By default, the long data buffer has zero initial capacity and is
     expanded to 4096 bytes when the first long data operation is performed.
     If your application requires a bigger or smaller long data buffer,
     you can specify a custom capacity using the <code>long_data_buffer()</code>
     accessor.</p>

  <p>The <code>mssql::connection_factory</code> abstract class has the
     following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace mssql
  {
    class connection_factory
    {
    public:
      virtual void
      database (database&amp;) = 0;

      virtual connection_ptr
      connect () = 0;
    };
  }
}
  </pre>

  <p>The <code>database()</code> function is called when a connection
     factory is associated with a database instance. This happens in
     the <code>odb::mssql::database</code> class constructors. The
     <code>connect()</code> function is called whenever a database
     connection is requested.</p>

  <p>The two implementations of the <code>connection_factory</code>
     interface provided by the SQL Server ODB runtime are
     <code>new_connection_factory</code> and
     <code>connection_pool_factory</code>. You will need to include
     the <code>&lt;odb/mssql/connection-factory.hxx></code>
     header file to make the <code>connection_factory</code> interface
     and these implementation classes available in your application.</p>

  <p>The <code>new_connection_factory</code> class creates a new
     connection whenever one is requested. When a connection is no
     longer needed, it is released and closed. The
     <code>new_connection_factory</code> class has the following
     interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace mssql
  {
    class new_connection_factory: public connection_factory
    {
    public:
      new_connection_factory ();
    };
};
  </pre>

  <p>The <code>connection_pool_factory</code> class implements a
     connection pool. It has the following interface:</p>

  <pre class="cxx">
namespace odb
{
  namespace mssql
  {
    class connection_pool_factory: public connection_factory
    {
    public:
      connection_pool_factory (std::size_t max_connections = 0,
                               std::size_t min_connections = 0);

    protected:
      class pooled_connection: public connection
      {
      public:
        pooled_connection (database_type&amp;);
        pooled_connection (database_type&amp;, SQLHDBC handle);
      };

      using pooled_connection_ptr = details::shared_ptr&lt;pooled_connection>;

      virtual pooled_connection_ptr
      create ();
    };
};
  </pre>

  <p>The <code>max_connections</code> argument in the
     <code>connection_pool_factory</code> constructor specifies the maximum
     number of concurrent connections that this pool factory will
     maintain. Similarly, the <code>min_connections</code> argument
     specifies the minimum number of available connections that
     should be kept open.</p>

  <p>Whenever a connection is requested, the pool factory first
     checks if there is an unused connection that can be returned.
     If there is none, the pool factory checks the
     <code>max_connections</code> value to see if a new connection
     can be created. If the total number of connections maintained
     by the pool is less than this value, then a new connection is
     created and returned. Otherwise, the caller is blocked until
     a connection becomes available.</p>

  <p>When a connection is released, the pool factory first checks
     if there are blocked callers waiting for a connection. If so, then
     one of them is unblocked and is given the connection. Otherwise,
     the pool factory checks whether the total number of connections
     maintained by the pool is greater than the <code>min_connections</code>
     value. If that's the case, the connection is closed. Otherwise, the
     connection is added to the pool of available connections to be
     returned on the next request. In other words, if the number of
     connections maintained by the pool exceeds <code>min_connections</code>
     and there are no callers waiting for a new connection,
     the pool will close the excess connections.</p>

  <p>If the <code>max_connections</code> value is 0, then the pool will
     create a new connection whenever all of the existing connections
     are in use. If the <code>min_connections</code> value is 0, then
     the pool will never close a connection and instead maintain all
     the connections that were ever created.</p>

  <p>The <code>create()</code> virtual function is called whenever the
     pool needs to create a new connection. By deriving from the
     <code>connection_pool_factory</code> class and overriding this
     function we can implement custom connection establishment
     and configuration.</p>

  <p>If you pass <code>NULL</code> as the connection factory to one of the
     <code>database</code> constructors, then the
     <code>connection_pool_factory</code> instance will be created by default
     with the min and max connections values set to <code>0</code>. The
     following code fragment shows how we can pass our own connection factory
     instance:</p>

  <pre class="cxx">
#include &lt;odb/database.hxx>

#include &lt;odb/mssql/database.hxx>
#include &lt;odb/mssql/connection-factory.hxx>

int
main (int argc, char* argv[])
{
  unique_ptr&lt;odb::mssql::connection_factory> f (
    new odb::mssql::connection_pool_factory (20));

  unique_ptr&lt;odb::database> db (
    new mssql::database (argc, argv, false, "", 0, f));
}
  </pre>

  <h2><a name="21.4">21.4 SQL Server Exceptions</a></h2>

  <p>The SQL Server ODB runtime library defines the following
     SQL Server-specific exceptions:</p>

  <pre class="cxx">
namespace odb
{
  namespace mssql
  {
    class database_exception: odb::database_exception
    {
    public:
      class record
      {
      public:
        SQLINTEGER
        error () const;

        const std::string&amp;
        sqlstate () const;

        const std::string&amp;
        message () const;
      };

      using records = std::vector&lt;record>;

      using size_type = records::size_type;
      using iterator = records::const_iterator;

      iterator
      begin () const;

      iterator
      end () const;

      size_type
      size () const;

      virtual const char*
      what () const throw ();
    };

    class cli_exception: odb::exception
    {
    public:
      virtual const char*
      what () const throw ();
    };

    class long_data_reload: odb::exception
    {
    public:
      virtual const char*
      what () const throw ();
    };
  }
}
  </pre>

  <p>You will need to include the <code>&lt;odb/mssql/exceptions.hxx></code>
     header file to make these exceptions available in your application.</p>

  <p>The <code>odb::mssql::database_exception</code> is thrown if
     an SQL Server database operation fails. The SQL Server-specific error
     information is stored as a series of records, each containing
     the error code as a signed 4-byte integer, the SQLSTATE code,
     and the message string. All this information is also combined
     and returned in a human-readable form by the <code>what()</code>
     function.</p>

  <p>The <code>odb::mssql::cli_exception</code> is thrown by the
     command line parsing constructor of the <code>odb::mssql::database</code>
     class if the SQL Server option values are missing or invalid. The
     <code>what()</code> function returns a human-readable description
     of an error.</p>

  <p>The <code>odb::mssql::long_data_reload</code> is thrown if an
     attempt is made to re-load an object or view with long data as
     part of a query result iteration. For more information, refer
     to <a href="#21.1">Section 21.1, "SQL Server Type Mapping"</a>.</p>

  <h2><a name="21.5">21.5 SQL Server Limitations</a></h2>

  <p>The following sections describe SQL Server-specific limitations imposed
     by the current SQL Server and ODB runtime versions.</p>

  <h3><a name="21.5.1">21.5.1 Query Result Caching</a></h3>

  <p>SQL Server ODB runtime implementation does not perform query result
     caching (<a href="#4.4">Section 4.4, "Query Result"</a>) even when
     explicitly requested. The ODBC API and the SQL Server Native Client ODBC
     driver support interleaving execution of multiple prepared statements
     on a single connection. As a result, it is possible to have multiple
     uncached results and calls to other database functions do not invalidate
     them. The only limitation of the uncached SQL Server results is the
     unavailability of the <code>result::size()</code> function. If you
     call this function on an SQL Server query result, then the
     <code>odb::result_not_cached</code> exception (<a href="#3.14">Section
     3.14, "ODB Exceptions"</a>) is always thrown. Future versions of the
     SQL Server ODB runtime library may add support for result caching.</p>

  <h3><a name="21.5.2">21.5.2 Foreign Key Constraints</a></h3>

  <p>ODB assumes the standard SQL behavior which requires that foreign
     key constraints checking is deferred until the transaction is
     committed. The only behavior supported by SQL Server is to check
     such constraints immediately. As a result, by default, schemas
     generated by the ODB compiler for SQL Server have foreign key
     definitions commented out. They are retained only for documentation.</p>

  <p>You can override the default behavior and instruct the ODB
     compiler to generate non-deferrable foreign keys by specifying
     the <code>--fkeys-deferrable-mode not_deferrable</code> ODB
     compiler option. Note, however, that in this case the order in
     which you persist, update, and erase objects within a transaction
     becomes important.</p>

  <h3><a name="21.5.3">21.5.3 Unique Constraint Violations</a></h3>

  <p>Due to the granularity of the ODBC error codes, it is impossible
     to distinguish between the duplicate primary key and other unique
     constraint violations. As a result, when making an object persistent,
     the SQL Server ODB runtime will translate all unique constraint violation
     errors to the <code>object_already_persistent</code> exception
     (<a href="#3.14">Section 3.14, "ODB Exceptions"</a>).</p>

  <h3><a name="21.5.4">21.5.4 Multi-threaded Windows Applications</a></h3>

  <p>Multi-threaded Windows applications must use the
     <code>_beginthread()</code>/<code>_beginthreadex()</code> and
     <code>_endthread()</code>/<code>_endthreadex()</code> CRT functions
     instead of the <code>CreateThread()</code> and <code>EndThread()</code>
     Win32 functions to start and terminate threads. This is a limitation of
     the ODBC implementation on Windows.</p>

  <h3><a name="21.5.5">21.5.5 Affected Row Count and DDL Statements</a></h3>

  <p>SQL Server always returns zero as the number of affected rows
     for DDL statements. In particular, this means that the
     <code>database::execute()</code> (<a href="#3.12">Section 3.12,
     "Executing Native SQL Statements"</a>) function will always
     return zero for such statements.</p>

  <h3><a name="21.5.6">21.5.6 Long Data and Auto Object Ids, <code>ROWVERSION</code></a></h3>

  <p>SQL Server 2005 has a bug that causes it to fail on an <code>INSERT</code>
     or <code>UPDATE</code> statement with the <code>OUTPUT</code> clause
     (used to return automatically assigned object ids as well as
      <code>ROWVERSION</code> values) if one of the inserted columns
     is long data. The symptom of this bug in ODB is an exception thrown
     by the <code>database::persist()</code> or <code>database::update()</code>
     function when used on an object that contains long data and has an
     automatically assigned object id or uses <code>ROWVERSION</code>-based
     optimistic concurrency (<a href="#21.1.1">Section 21.1.1,
     "<code>ROWVERSION</code> Support"</a>). The error message reads "This
     operation conflicts with another pending operation on this transaction.
     The operation failed."</p>

  <p>For automatically assigned object ids ODB includes a workaround for
     this bug which uses a less efficient method to obtain id values for
     objects that contain long data. To enable this workaround you need
     to specify that the generated code will be used with SQL Server 2005
     or later by passing the <code>--mssql-server-version&nbsp;9.0</code>
     ODB compiler option.</p>

  <p>For <code>ROWVERSION</code>-based optimistic concurrency no workaround
     is currently provided. The ODB compiler will issue an error for
     objects that use <code>ROWVERSION</code> for optimistic concurrency
     and containing long data.</p>

  <h3><a name="21.5.7">21.5.7 Long Data and By-Value Accessors/Modifiers</a></h3>

  <p>As discussed in <a href="#14.4.5">Section 14.4.5,
     "<code>get</code>/<code>set</code>/<code>access</code>"</a>, by-value
     accessor and modifier expressions cannot be used with data members
     of long data types. The SQL Server ODB runtime uses streaming for
     reading/writing long data directly from/to data members. As a result,
     by-reference accessors and modifiers should be used for these data
     types.</p>

  <h3><a name="21.5.8">21.5.8 Bulk Update and <code>ROWVERSION</code></a></h3>

  <p>The bulk update operation (<a href="#15.3">Section 15.3, "Bulk Database
     Operations"</a>) is not yet supported for persistent classes that use
     <code>ROWVERSION</code>-based optimistic concurrency. For such classes
     the bulk <code>update()</code> function is not available. The bulk
     persist and erase support is still provided.</p>

  <h2><a name="21.6">21.6 SQL Server Index Definitions</a></h2>

  <p>When the <code>index</code> pragma (<a href="#14.7">Section 14.7,
     "Index Definition Pragmas"</a>) is used to define an SQL Server index,
     the <code>type</code> clause specifies the index type (for example,
     <code>UNIQUE</code>, <code>CLUSTERED</code>), the <code>method</code>
     clause is not used, and the <code>options</code> clause specifies
     additional index properties. The column options can be used to specify
     the sort order. For example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  std::string name_;

  #pragma db index                             \
             type("UNIQUE CLUSTERED")          \
             member(name_, "DESC")             \
             options("WITH(FILLFACTOR = 80)")
};
  </pre>

  <h2><a name="21.7">21.7 SQL Server Stored Procedures</a></h2>

  <p>ODB native views (<a href="#10.6">Section 10.6, "Native Views"</a>)
     can be used to call SQL Server stored procedures. For example, assuming
     we are using the <code>person</code> class from <a href="#2">Chapter
     2, "Hello World Example"</a> (and the corresponding <code>person</code>
     table), we can create a stored procedure that given the min and max
     ages returns some information about all the people in that range:</p>

  <pre class="sql">
CREATE PROCEDURE dbo.person_range (
  @min_age SMALLINT,
  @max_age SMALLINT)
AS
  SELECT age, first, last FROM person
    WHERE age >= @min_age AND age &lt;= @max_age;
  </pre>

  <p>Given the above stored procedure we can then define an ODB view
     that can be used to call it and retrieve its result:</p>

  <pre class="cxx">
#pragma db view query("EXEC person_range (?)")
struct person_range
{
  unsigned short age;
  std::string first;
  std::string last;
};
  </pre>

  <p>The following example shows how we can use the above view to
     print the list of people in a specific age range:</p>

  <pre class="cxx">
using query = odb::query&lt;person_range>;
using result = odb::result&lt;person_range>;

transaction t (db.begin ());

result r (
  db.query&lt;person_range> (
    query::_val (1) + "," + query::_val (18)));

for (result::iterator i (r.begin ()); i != r.end (); ++i)
  cerr &lt;&lt; i->first &lt;&lt; " " &lt;&lt; i->last &lt;&lt; " " &lt;&lt; i->age &lt;&lt; endl;

t.commit ();
  </pre>

   <p>Note that as with all native views, the order and types of data members
      must match those of columns in the <code>SELECT</code> list inside
      the stored procedure.</p>

   <p>There are also a number of limitations when it comes to calling
      SQL Server stored procedures with ODB views. There is currently
      no support for output parameters, however, this is planned for
      a future version. In the meantime, to call a stored procedure
      that has output parameters we have to use a wrapper procedure
      that converts such parameters to a <code>SELECT</code>
      result. For example, given the following procedure that
      calculates the age range of the people in our database:</p>

   <pre class="sql">
CREATE PROCEDURE dbo.person_age_range (
  @min_age SMALLINT = NULL OUTPUT,
  @max_age SMALLINT = NULL OUTPUT)
AS
  SELECT @min_age = MIN(age), @max_age = MAX(max) FROM person;
  </pre>

   <p>We can create a wrapper procedure like this:</p>

   <pre class="sql">
CREATE PROCEDURE dbo.person_age_range_odb
AS
  DECLARE @min_age SMALLINT, @max_age SMALLINT;
  EXEC person_age_range @min_age OUTPUT, @max_age OUTPUT;
  SELECT @min_age, @max_age;
   </pre>

   <p>And a view like this:</p>

   <pre class="cxx">
#pragma db view query("EXEC person_age_range_odb")
struct person_age_range
{
  unsigned short min_age;
  unsigned short max_age;
};
  </pre>

  <p>Which we can then use to call the stored procedure:</p>

  <pre class="cxx">
transaction t (db.begin ());

person_age_range ar (db.query_value&lt;person_age_range> ());
cerr &lt;&lt; ar.min_age &lt;&lt; " " &lt;&lt; ar.max_age &lt;&lt; endl;

t.commit ();
  </pre>

  <p>In SQL Server, a stored procedure can produce multiple results.
     For example, if a stored procedure executes several
     <code>SELECT</code> statements, then the result of calling such a
     procedure consists of multiple row sets, one for each <code>SELECT</code>
     statement. Because such multiple row sets can contain varying number and
     type of columns, they cannot be all extracted into a single
     view. Consequently, these kind of stored procedures are currently not
     supported.</p>

  <p>A stored procedure may also produce no row sets at all. For
     example, a stored procedure that only executes DML statements
     would exhibit this behavior. To call such a procedure we use
     an empty view, for example:</p>

  <pre class="sql">
CREATE PROCEDURE dbo.insert_person (
  @first VARCHAR(512),
  @last VARCHAR(512),
  @age SMALLINT)
AS
  INSERT INTO person(first, last, age)
    VALUES(@first, @last, @age);
  </pre>

  <pre class="cxx">
#pragma db view
struct no_result {};

transaction t (db.begin ());

db.query_one&lt;no_result> (
  "EXEC insert_person" +
    query::_val ("John") + "," +
    query::_val ("Doe") + "," +
    query::_val (21));

t.commit ();
  </pre>

  <p>Finally, an SQL Server stored procedure can also return an
     integer status code. Similar to output parameters, this code
     can only be observed by an ODB view if it is converted to a
     <code>SELECT</code> result. For more information on how to
     do this and for other examples of stored procedure calls,
     refer to the <code>mssql/stored-proc</code> test in the
     <code>odb-tests</code> package.</p>

  <!-- PART -->


  <hr class="page-break"/>
  <h1><a name="III">PART III&nbsp;&nbsp;
      <span style="font-weight: normal;">PROFILES</span></a></h1>

  <p>Part III covers the integration of ODB with popular C++ frameworks
     and libraries. It consists of the following chapters.</p>

  <table class="toc">
    <tr><th>22</th><td><a href="#22">Profiles Introduction</a></td></tr>
    <tr><th>23</th><td><a href="#23">Boost Profile</a></td></tr>
    <tr><th>24</th><td><a href="#24">Qt Profile</a></td></tr>
  </table>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="22">22 Profiles Introduction</a></h1>

  <p>ODB profiles are a generic mechanism for integrating ODB with
     widely-used C++ frameworks and libraries. A profile provides glue
     code which allows you to seamlessly persist various components, such
     as smart pointers, containers, and value types found in these
     frameworks or libraries. The code necessary to implement a profile
     is packaged into the so called profile library. For example, the
     Boost profile implementation is provided by the <code>libodb-boost</code>
     profile library.</p>

  <p>Besides linking the profile library to our application, it is also
     necessary to let the ODB compiler know which profiles we
     are using. This is accomplished with the <code>--profile</code>
     (or <code>-p</code> alias) option. For example:</p>

  <pre class="terminal">
odb --profile boost ...
  </pre>

  <p>Some profiles, especially those covering frameworks or libraries that
     consist of multiple sub-libraries, provide sub-profiles that allow you
     to pick and choose which components you would like to use in your
     application. For example, the <code>boost</code> profile contains
     the <code>boost/data-time</code> sub-profile. If we are only
     interested in the <code>date_time</code> types, then we can
     pass <code>boost/data-time</code> instead of <code>boost</code>
     to the <code>--profile</code> option, for example:</p>

  <pre class="terminal">
odb --profile boost/date-time ...
  </pre>

  <p>To summarize, you will need to perform the following steps in order
     to make use of a profile in your application:</p>

  <ol>
    <li>ODB compiler: if necessary, specify the path to the profile library
        headers (<code>-I</code> option).</li>
    <li>ODB compiler: specify the profile you would like to use with
        the <code>--profile</code> option.</li>
    <li>C++ compiler: if necessary, specify the path to the profile library
        headers (normally <code>-I</code> option).</li>
    <li>Linker: link the profile library to the application.</li>
  </ol>

  <p>The remaining chapters in this part of the manual describe the
     standard profiles provided by ODB.</p>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="23">23 Boost Profile</a></h1>

  <p>The ODB profile implementation for Boost is provided by the
     <code>libodb-boost</code> library and consists of multiple sub-profiles
     corresponding to the individual Boost libraries. To enable all the
     available Boost sub-profiles, pass <code>boost</code> as the profile
     name to the <code>--profile</code> ODB compiler option. Alternatively,
     you can enable only specific sub-profiles by passing individual
     sub-profile names to <code>--profile</code>. The following sections in
     this chapter discuss each Boost sub-profile in detail. The
     <code>boost</code> example in the <code>odb-examples</code>
     package shows how to enable and use the Boost profile.</p>

  <p>Some sub-profiles may throw exceptions to indicate error conditions,
     such as the inability to store a specific value in a particular database
     system. All such exceptions derive from the
     <code>odb::boost::exception</code> class which in turn derives from
     the root of the ODB exception hierarchy, class <code>odb::exception</code>
     (<a href="#3.14">Section 3.14, "ODB Exceptions"</a>). The
     <code>odb::boost::exception</code> class is defined in the
     <code>&lt;odb/boost/exception.hxx></code> header file and has the
     same interface as <code>odb::exception</code>. Concrete exceptions
     that can be thrown by the Boost sub-profiles are described in the
     following sections.</p>

  <h2><a name="23.1">23.1 Smart Pointers Library</a></h2>

  <p>The <code>smart-ptr</code> sub-profile provides persistence
     support for a subset of smart pointers from the Boost
     <code>smart_ptr</code> library. To enable only this profile,
     pass <code>boost/smart-ptr</code> to the <code>--profile</code>
     ODB compiler option.</p>

  <p>The currently supported smart pointers are
     <code>boost::shared_ptr</code> and <code>boost::weak_ptr</code>. For
     more information on using smart pointers as pointers to objects and
     views, refer to <a href="#3.3">Section 3.3, "Object and View Pointers"</a>
     and <a href="#6">Chapter 6, "Relationships"</a>. For more information
     on using smart pointers as pointers to values, refer to
     <a href="#7.3">Section 7.3, "Pointers and <code>NULL</code> Value
     Semantics"</a>. When used as a pointer to a value, only
     <code>boost::shared_ptr</code> is supported. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db null
  boost::shared_ptr&lt;std::string> middle_name_;
};
  </pre>

  <p>To provide finer grained control over object relationship loading,
     the <code>smart-ptr</code> sub-profile also provides the lazy
     counterparts for the above  pointers: <code>odb::boost::lazy_shared_ptr</code> and
     <code>odb::boost::lazy_weak_ptr</code>. You will need to include the
     <code>&lt;odb/boost/lazy-ptr.hxx></code> header file to make the lazy
     variants available in your application. For a description of the lazy
     pointer interface and semantics refer to <a href="#6.4">Section 6.4,
     "Lazy Pointers"</a>. The following example shows how we can use these
     smart pointers to establish a relationship between persistent objects.</p>

  <pre class="cxx">
class employee;

#pragma db object
class position
{
  ...

  #pragma db inverse(position_)
  odb::boost::lazy_weak_ptr&lt;employee> employee_;
};

#pragma db object
class employee
{
  ...

  #pragma db not_null
  boost::shared_ptr&lt;position> position_;
};
  </pre>

  <p>Besides providing persistence support for the above smart pointers,
     the <code>smart-ptr</code> sub-profile also changes the default
     pointer (<a href="#3.3">Section 3.3, "Object and View Pointers"</a>)
     to <code>boost::shared_ptr</code>. In particular, this means that
     database functions that return dynamically allocated objects and views
     will return them as <code>boost::shared_ptr</code> pointers.  To override
     this behavior, add the <code>--default-pointer</code> option specifying
     the alternative pointer type after the <code>--profile</code> option.</p>

  <h2><a name="23.2">23.2 Unordered Containers Library</a></h2>

  <p>The <code>unordered</code> sub-profile provides persistence support for
     the containers from the Boost <code>unordered</code> library. To enable
     only this profile, pass <code>boost/unordered</code> to
     the <code>--profile</code> ODB compiler option.</p>

  <p>The supported containers are <code>boost::unordered_set</code>,
     <code>boost::unordered_map</code>, <code>boost::unordered_multiset</code>,
     and <code>boost::unordered_multimap</code>. For more information on using
     the set and multiset containers with ODB, refer to <a href="#5.2">Section
     5.2, "Set and Multiset Containers"</a>. For more information on using the
     map and multimap containers with ODB, refer to <a href="#5.3"> Section
     5.3, "Map and Multimap Containers"</a>. The following example shows how
     the <code>unordered_set</code> container may be used within a persistent
     object.</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
  boost::unordered_set&lt;std::string&gt; emails_;
};
  </pre>

  <h2><a name="23.3">23.3 Multi-Index Container Library</a></h2>

  <p>The <code>multi-index</code> sub-profile provides persistence support for
     <code>boost::multi_index_container</code> from the Boost Multi-Index
     library. To enable only this profile, pass <code>boost/multi-index</code>
     to the <code>--profile</code> ODB compiler option. The following example
     shows how <code>multi_index_container</code> may be used within a
     persistent object.</p>

  <pre class="cxx">
namespace mi = boost::multi_index;

#pragma db object
class person
{
  ...

  using emails =
    mi::multi_index_container&lt;
      std::string,
      mi::indexed_by&lt;
        mi::sequenced&lt;>,
        mi::ordered_unique&lt;mi::identity&lt;std::string>>
      >
    >;

  emails emails_;
};
  </pre>

  <p>Note that a <code>multi_index_container</code> instantiation is
     stored differently in the database depending on whether it has
     any <code>sequenced</code> or <code>random_access</code> indexes.
     If it does, then it is treated as an ordered container
     (<a href="#5.1">Section 5.1, "Ordered Containers"</a>) with the
     first such index establishing the order. Otherwise, it is treated
     as a set container (<a href="#5.2">Section 5.2, "Set and Multiset
     Containers"</a>).</p>

  <p>Note also that there is a terminology clash between ODB and Boost
     Multi-Index. The ODB term <em>ordered container</em> translates
     to Multi-Index terms <em>sequenced index</em> and <em>random access
     index</em> while the ODB term <em>set container</em> translates
     to Multi-Index terms <em>ordered index</em> and <em>hashed
     index</em>.</p>

  <p>The <code>emails</code> container from the above example is stored
     as an ordered container. In contrast, the following <code>aliases</code>
     container is stored as a set.</p>

  <pre class="cxx">
namespace mi = boost::multi_index;

#pragma db value
struct name
{
  std::string first;
  std::string last;
};

bool operator&lt; (const name&amp;, const name&amp;);

#pragma db object
class person
{
  ...

  using aliases =
    mi::multi_index_container&lt;
      name,
      mi::indexed_by&lt;
        mi::ordered_unique&lt;mi::identity&lt;name>>
        mi::ordered_non_unique&lt;
          mi::member&lt;name, std::string, &amp;name::first>
        >,
        mi::ordered_non_unique&lt;
          mi::member&lt;name, std::string, &amp;name::last>
        >
      >
    > ;

  aliases aliases_;
};
  </pre>

  <h2><a name="23.4">23.4 Optional Library</a></h2>

  <p>The <code>optional</code> sub-profile provides persistence support for
     the <code>boost::optional</code> container from the Boost
     <code>optional</code> library. To enable only this profile, pass
     <code>boost/optional</code> to the <code>--profile</code> ODB compiler
     option.</p>

  <p>In a relational database <code>boost::optional</code> is mapped to
     a column that can have a <code>NULL</code> value. Similar to
     <code>odb::nullable</code> (<a href="#7.3">Section 7.3, "Pointers and
     <code>NULL</code> Value Semantics"</a>), it can be used to add the
     <code>NULL</code> semantics to existing C++ types. For example:</p>

  <pre class="cxx">
#include &lt;boost/optional.hpp>

#pragma db object
class person
{
  ...

  std::string first_;                    // TEXT NOT NULL
  boost::optional&lt;std::string> middle_;  // TEXT NULL
  std::string last_;                     // TEXT NOT NULL
};
  </pre>

  <p>Note also that similar to <code>odb::nullable</code>, when
     this profile is used, the <code>NULL</code> values are automatically
     enabled for data members of the  <code>boost::optional</code> type.</p>

  <h2><a name="23.5">23.5 Date Time Library</a></h2>

  <p>The <code>date-time</code> sub-profile provides persistence support for a
     subset of types from the Boost <code>date_time</code> library. It is
     further subdivided into two sub-profiles, <code>gregorian</code>
     and <code>posix_time</code>. The <code>gregorian</code> sub-profile
     provides support for types from the <code>boost::gregorian</code>
     namespace, while the <code>posix-time</code> sub-profile provides support
     for types from the <code>boost::posix_time</code> namespace. To enable
     the entire <code>date-time</code> sub-profile, pass
     <code>boost/date-time</code> to the <code>--profile</code> ODB compiler
     option. To enable only the <code>gregorian</code> sub-profile, pass
     <code>boost/date-time/gregorian</code>, and to enable only the
     <code>posix-time</code> sub-profile, pass
     <code>boost/date-time/posix-time</code>.</p>

  <p>The only type that the <code>gregorian</code> sub-profile currently
     supports is <code>gregorian::date</code>. The types currently supported
     by the <code>posix-time</code> sub-profile are
     <code>posix_time::ptime</code> and
     <code>posix_time::time_duration</code>. The manner in which these types
     are persisted is database system dependent and is discussed in the
     sub-sections that follow. The example below shows how
     <code>gregorian::date</code> may be used within a persistent object.</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
  boost::gregorian::date date_of_birth_;
};
  </pre>

  <p>Concrete exceptions that can be thrown by the <code>date-time</code>
     sub-profile implementation are presented below.</p>


  <pre class="cxx">
namespace odb
{
  namespace boost
  {
    namespace date_time
    {
      struct special_value: odb::boost::exception
      {
        virtual const char*
        what () const throw ();
      };

      struct value_out_of_range: odb::boost::exception
      {
        virtual const char*
        what () const throw ();
      };
    }
  }
}
  </pre>

  <p>You will need to include the
     <code>&lt;odb/boost/date-time/exceptions.hxx&gt;</code> header file to
     make these exceptions available in your application.</p>

  <p>The <code>special_value</code> exception is thrown if an attempt is made
     to store a Boost date-time special value that cannot be represented in
     the target database. The <code>value_out_of_range</code> exception is
     thrown if an attempt is made to store a date-time value that is out of
     the target database range. The specific conditions under which these
     exceptions are thrown are database system dependent and are discussed in
     more detail in the following sub-sections.</p>

  <h3><a name="23.5.1">23.5.1 MySQL Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported Boost <code>date_time</code> types and the MySQL database
     types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Boost <code>date_time</code> Type</th>
      <th>MySQL Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>gregorian::date</code></td>
      <td><code>DATE</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>posix_time::ptime</code></td>
      <td><code>DATETIME</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>posix_time::time_duration</code></td>
      <td><code>TIME</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>The Boost special value <code>date_time::not_a_date_time</code> is stored
     as a <code>NULL</code> value in a MySQL database.</p>

  <p>The <code>posix-time</code> sub-profile implementation also provides
     support for mapping <code>posix_time::ptime</code> to the
     <code>TIMESTAMP</code> MySQL type. However, this mapping has to be
     explicitly requested using the <code>db&nbsp;type</code> pragma
     (<a href="#14.4.3">Section 14.4.3, "<code>type</code>"</a>), as shown in
     the following example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
  #pragma db type("TIMESTAMP") not_null
  boost::posix_time::ptime updated_;
};
  </pre>

  <p>Starting with MySQL version 5.6.4 it is possible to store fractional
     seconds up to microsecond precision in <code>TIME</code>,
     <code>DATETIME</code>, and <code>TIMESTAMP</code> columns. However,
     to enable sub-second precision, the corresponding type with the
     desired precision has to be specified explicitly, as shown in the
     following example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
  #pragma db type("DATETIME(6)")     // Microsecond precision.
  boost::posix_time::ptime updated_;
};
  </pre>

  <p>Alternatively, you can enable sub-second precision on the per-type
     basis, for example:</p>

  <pre class="cxx">
#pragma db value(boost::posix_time::ptime) type("DATETIME(6)")

#pragma db object
class person
{
  ...
  boost::posix_time::ptime created_; // Microsecond precision.
  boost::posix_time::ptime updated_; // Microsecond precision.
};
  </pre>

  <p>Some valid Boost date-time values cannot be stored in a MySQL database.
     An attempt to persist any Boost date-time special value other than
     <code>date_time::not_a_date_time</code> will result in the
     <code>special_value</code> exception. An attempt to persist a Boost
     date-time value that is out of the MySQL type range will result in
     the <code>out_of_range</code> exception.  Refer to the MySQL
     documentation for more information on the MySQL data type ranges.</p>

  <h3><a name="23.5.2">23.5.2 SQLite Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported Boost <code>date_time</code> types and the SQLite database
     types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Boost <code>date_time</code> Type</th>
      <th>SQLite Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>gregorian::date</code></td>
      <td><code>TEXT</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>posix_time::ptime</code></td>
      <td><code>TEXT</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>posix_time::time_duration</code></td>
      <td><code>TEXT</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>The Boost special value <code>date_time::not_a_date_time</code> is stored
     as a <code>NULL</code> value in an SQLite database.</p>

  <p>The <code>date-time</code> sub-profile implementation also provides
     support for mapping <code>gregorian::date</code> and
     <code>posix_time::ptime</code> to the <code>INTEGER</code> SQLite type,
     with the integer value representing the UNIX time. Similarly, an
     alternative mapping for <code>posix_time::time_duration</code> to the
     <code>INTEGER</code> type represents the duration as a number of
     seconds. These mappings have to be explicitly requested using the
     <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section 14.4.3,
     "<code>type</code>"</a>), as shown in the following example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
  #pragma db type("INTEGER")
  boost::gregorian::date born_;
};
  </pre>

  <!--

  <p>The Boost UNIX time interface does not support 64 bit time arithmetic.
     As a result, the UNIX time representations of <code>gregorian::date</code>
     and <code>posix_time::ptime</code> are restricted to the 32 bit range.
     The minimum and maximum date representable by
     <code>gregorian::date</code> is 1901-12-14 and 2038-01-19 respectively,
     while the minimum and maximum date-time representable by
     <code>posix_time::ptime</code> is 1901-12-13&nbsp;20:45:54 GMT and
     2038-01-19&nbsp;03:14:07&nbsp;GMT respectively. Persisting and loading
     of values outside of these ranges will result in undefined behavior.</p>

  -->

  <p>Some valid Boost date-time values cannot be stored in an SQLite database.
     An attempt to persist any Boost date-time special value other than
     <code>date_time::not_a_date_time</code> will result in the
     <code>special_value</code> exception. An attempt to persist a negative
     <code>posix_time::time_duration</code> value as SQLite <code>TEXT</code>
     will result in the <code>out_of_range</code> exception.</p>


  <h3><a name="23.5.3">23.5.3 PostgreSQL Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported Boost <code>date_time</code> types and the PostgreSQL database
     types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Boost <code>date_time</code> Type</th>
      <th>PostgreSQL Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>gregorian::date</code></td>
      <td><code>DATE</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>posix_time::ptime</code></td>
      <td><code>TIMESTAMP</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>posix_time::time_duration</code></td>
      <td><code>TIME</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>The Boost special value <code>date_time::not_a_date_time</code> is stored
     as a <code>NULL</code> value in a PostgreSQL database.
     <code>posix_time::ptime</code> values representing the special values
     <code>date_time::pos_infin</code> and <code>date_time::neg_infin</code>
     are stored as the special PostgreSQL TIMESTAMP values
     <code>infinity</code> and <code>-infinity</code>, respectively.</p>

  <p>Some valid Boost date-time values cannot be stored in a PostgreSQL
     database. The PostgreSQL TIME type represents a clock time, and can
     therefore only store positive durations with a total length of time less
     than 24 hours. An attempt to persist a
     <code>posix_time::time_duration</code> value outside of this range will
     result in the <code>value_out_of_range</code> exception. An attempt to
     persist a <code>posix_time::time_duration</code> value representing any
     special value other than <code>date_time::not_a_date_time</code>  will
     result in the <code>special_value</code> exception.</p>


  <h3><a name="23.5.4">23.5.4 Oracle Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported Boost <code>date_time</code> types and the Oracle database
     types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Boost <code>date_time</code> Type</th>
      <th>Oracle Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>gregorian::date</code></td>
      <td><code>DATE</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>posix_time::ptime</code></td>
      <td><code>TIMESTAMP</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>posix_time::time_duration</code></td>
      <td><code>INTERVAL DAY TO SECOND</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>The Boost special value <code>date_time::not_a_date_time</code> is stored
     as a <code>NULL</code> value in an Oracle database.</p>

  <p>The <code>date-time</code> sub-profile implementation also provides
     support for mapping <code>posix_time::ptime</code> to the
     <code>DATE</code> Oracle type with fractional seconds that may be
     stored in a <code>ptime</code> instance being ignored. This
     alternative mapping has to be explicitly requested using the
     <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section 14.4.3,
     "<code>type</code>"</a>), as shown in the following example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
  #pragma db type("DATE")
  boost::posix_time::ptime updated_;
};
  </pre>

  <p>Some valid Boost date-time values cannot be stored in an Oracle database.
     An attempt to persist a <code>gregorian::date</code>,
     <code>posix_time::ptime</code>, or
     <code>posix_time::time_duration</code> value representing any special
     value other than <code>date_time::not_a_date_time</code> will result in
     the <code>special_value</code> exception.</p>


  <h3><a name="23.5.5">23.5.5 SQL Server Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported Boost <code>date_time</code> types and the SQL Server database
     types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Boost <code>date_time</code> Type</th>
      <th>SQL Server Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>gregorian::date</code></td>
      <td><code>DATE</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>posix_time::ptime</code></td>
      <td><code>DATETIME2</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>posix_time::time_duration</code></td>
      <td><code>TIME</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>The Boost special value <code>date_time::not_a_date_time</code> is stored
     as a <code>NULL</code> value in an SQL Server database.</p>

  <p>Note that the <code>DATE</code>, <code>TIME</code>, and
     <code>DATETIME2</code> types are only available in SQL Server 2008 and
     later. SQL Server 2005 only supports the <code>DATETIME</code> and
     <code>SMALLDATETIME</code> date-time types. The new types are
     also unavailable when connecting to an SQL Server 2008 or
     later with the SQL Server 2005 Native Client ODBC driver.</p>

  <p>The <code>date-time</code> sub-profile implementation provides
     support for mapping <code>posix_time::ptime</code> to the
     <code>DATETIME</code> and <code>SMALLDATETIME</code> types,
     however, this mapping has to be explicitly requested using the
     <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section 14.4.3,
     "<code>type</code>"</a>), as shown in the following example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
  #pragma db type("DATETIME")
  boost::posix_time::ptime updated_;
};
  </pre>

  <p>Some valid Boost date-time values cannot be stored in an SQL Server
     database. An attempt to persist a <code>gregorian::date</code>,
     <code>posix_time::ptime</code>, or <code>posix_time::time_duration</code>
     value representing any special value other than
     <code>date_time::not_a_date_time</code> will result in the
     <code>special_value</code> exception. The range of the <code>TIME</code>
     type in SQL server is from <code>00:00:00.0000000</code> to
     <code>23:59:59.9999999</code>. An attempt to persist a
     <code>posix_time::time_duration</code> value out of this range will
     result in the <code>value_out_of_range</code> exception.</p>

  <h2><a name="23.6">23.6 Uuid Library</a></h2>

  <p>The <code>uuid</code> sub-profile provides persistence support for the
     <code>uuid</code> type from the Boost <code>uuid</code> library. To
     enable only this profile, pass <code>boost/uuid</code> to the
     <code>--profile</code> ODB compiler option.</p>

  <p>The manner in which these types are persisted is database system
     dependent and is discussed in the sub-sections that follow. By
     default a data member of the <code>uuid</code> type is mapped to a
     database column with <code>NULL</code> enabled and nil <code>uuid</code>
     instances are stored as a <code>NULL</code> value. However, you can
     change this behavior by declaring the data member <code>NOT NULL</code>
     with the <code>not_null</code> pragma (<a href="#14.4.6">Section
     14.4.6, "<code>null</code>/<code>not_null</code>"</a>). In this
     case, or if the data member is an object id, the implementation
     will store nil <code>uuid</code> instances as zero UUID values
     (<code>{00000000-0000-0000-0000-000000000000}</code>). For example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  boost::uuids::uuid x_; // Nil values stored as NULL.

  #pragma db not_null
  boost::uuids::uuid y_; // Nil values stored as zero.
};
  </pre>

  <h3><a name="23.6.1">23.6.1 MySQL Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the Boost
     <code>uuid</code> type and the MySQL database type.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Boost Type</th>
      <th>MySQL Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>boost::uuids::uuid</code></td>
      <td><code>BINARY(16)</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <h3><a name="23.6.2">23.6.2 SQLite Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the Boost
     <code>uuid</code> type and the SQLite database type.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Boost Type</th>
      <th>SQLite Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>boost::uuids::uuid</code></td>
      <td><code>BLOB</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <h3><a name="23.6.3">23.6.3 PostgreSQL Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the Boost
     <code>uuid</code> type and the PostgreSQL database type.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Boost Type</th>
      <th>PostgreSQL Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>boost::uuids::uuid</code></td>
      <td><code>UUID</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <h3><a name="23.6.4">23.6.4 Oracle Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the Boost
     <code>uuid</code> type and the Oracle database type.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Boost Type</th>
      <th>Oracle Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>boost::uuids::uuid</code></td>
      <td><code>RAW(16)</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <h3><a name="23.6.5">23.6.5 SQL Server Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the Boost
     <code>uuid</code> type and the SQL Server database type.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Boost Type</th>
      <th>SQL Server Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>boost::uuids::uuid</code></td>
      <td><code>UNIQUEIDENTIFIER</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>


  <!-- CHAPTER -->


  <hr class="page-break"/>
  <h1><a name="24">24 Qt Profile</a></h1>

  <p>The ODB profile implementation for Qt is provided by the
     <code>libodb-qt</code> library. Both Qt4 and Qt5 as well
     as C++98/03 and C++11 are supported.</p>

  <p>The Qt profile consists of multiple sub-profiles
     corresponding to the common type groups within Qt. Currently,
     only types from the <code>QtCore</code> module are supported. To
     enable all the available Qt sub-profiles, pass <code>qt</code> as the
     profile name to the <code>--profile</code> ODB compiler  option.
     Alternatively, you can enable only specific sub-profiles by passing
     individual sub-profile names to <code>--profile</code>. The following
     sections in this chapter discuss each Qt sub-profile in detail. The
     <code>qt</code> example in the <code>odb-examples</code>
     package shows how to enable and use the Qt profile.</p>

  <p>Some sub-profiles may throw exceptions to indicate error conditions,
     such as the inability to store a specific value in a particular database
     system. All such exceptions derive from the
     <code>odb::qt::exception</code> class which in turn derives from
     the root of the ODB exception hierarchy, class <code>odb::exception</code>
     (<a href="#3.14">Section 3.14, "ODB Exceptions"</a>). The
     <code>odb::qt::exception</code> class is defined in the
     <code>&lt;odb/qt/exception.hxx></code> header file and has the
     same interface as <code>odb::exception</code>. Concrete exceptions
     that can be thrown by the Qt sub-profiles are described in the
     following sections.</p>

  <h2><a name="24.1">24.1 Basic Types Library</a></h2>

  <p>The <code>basic</code> sub-profile provides persistence support for basic
     types defined by Qt. To enable only this profile, pass
     <code>qt/basic</code> to the <code>--profile</code> ODB compiler
     option.</p>

  <p>The currently supported basic types are <code>QString</code>,
    <code>QByteArray</code>, and <code>QUuid</code>. The manner in
     which these types are persisted is database system dependent
     and is discussed in the sub-sections that follow. The example
     below shows how <code>QString</code> may be used within a
     persistent object.</p>

  <pre class="cxx">
#pragma db object
class Person
{
  ...
  QString name_;
};
  </pre>

  <p>By default a data member of the <code>QUuid</code> type is mapped to a
     database column with <code>NULL</code> enabled and null <code>QUuid</code>
     instances are stored as a <code>NULL</code> value. However, you can
     change this behavior by declaring the data member <code>NOT NULL</code>
     with the <code>not_null</code> pragma (<a href="#14.4.6">Section
     14.4.6, "<code>null</code>/<code>not_null</code>"</a>). In this
     case, or if the data member is an object id, the implementation
     will store null <code>QUuid</code> instances as zero UUID values
     (<code>{00000000-0000-0000-0000-000000000000}</code>). For example:</p>

  <pre class="cxx">
#pragma db object
class object
{
  ...

  QUuid x_; // Null values stored as NULL.

  #pragma db not_null
  QUuid y_; // Null values stored as zero.
};
  </pre>

  <h3><a name="24.1.1">24.1.1 MySQL Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported basic Qt types and the MySQL database types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Qt Type</th>
      <th>MySQL Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>QString</code></td>
      <td><code>TEXT/VARCHAR(128)</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QByteArray</code></td>
      <td><code>BLOB</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QUuid</code></td>
      <td><code>BINARY(16)</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>Instances of the <code>QString</code> and <code>QByteArray</code>
     types are stored as a <code>NULL</code> value if their
     <code>isNull()</code> member function returns <code>true</code>.</p>

  <p>Note also that the <code>QString</code> type is mapped
     differently depending on whether a member of this type
     is an object id or not. If the member is an object id,
     then for this member <code>QString</code> is mapped
     to the <code>VARCHAR(128)</code> MySQL type. Otherwise,
     it is mapped to <code>TEXT</code>.</p>

  <p>The <code>basic</code> sub-profile also provides support
     for mapping <code>QString</code> to the <code>CHAR</code>,
     <code>NCHAR</code>, and <code>NVARCHAR</code> MySQL types.
     However, these alternative mappings have to be explicitly
     requested using the <code>db&nbsp;type</code> pragma
     (<a href="#14.4.3">Section 14.4.3, "type"</a>), as shown in
     the following example:</p>

  <pre class="cxx">
#pragma db object
class Person
{
  ...

  #pragma db type("CHAR(2)") not_null
  QString licenseState_;
};
  </pre>


  <h3><a name="24.1.2">24.1.2 SQLite Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported basic Qt types and the SQLite database types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Qt Type</th>
      <th>SQLite Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>QString</code></td>
      <td><code>TEXT</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QByteArray</code></td>
      <td><code>BLOB</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QUuid</code></td>
      <td><code>BLOB</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>Instances of the <code>QString</code> and <code>QByteArray</code> types
     are stored as a <code>NULL</code> value if their <code>isNull()</code>
     member function returns <code>true</code>.</p>

  <h3><a name="24.1.3">24.1.3 PostgreSQL Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported basic Qt types and the PostgreSQL database types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Qt Type</th>
      <th>PostgreSQL Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>QString</code></td>
      <td><code>TEXT</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QByteArray</code></td>
      <td><code>BYTEA</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QUuid</code></td>
      <td><code>UUID</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>Instances of the <code>QString</code> and <code>QByteArray</code> types
     are stored as a <code>NULL</code> value if their <code>isNull()</code>
     member function returns <code>true</code>.</p>

  <p>The <code>basic</code> sub-profile also provides support
     for mapping <code>QString</code> to the <code>CHAR</code>
     and <code>VARCHAR</code> PostgreSQL types.
     However, these alternative mappings have to be explicitly
     requested using the <code>db&nbsp;type</code> pragma
     (<a href="#14.4.3">Section 14.4.3, "type"</a>), as shown in
     the following example:</p>

  <pre class="cxx">
#pragma db object
class Person
{
  ...

  #pragma db type("CHAR(2)") not_null
  QString licenseState_;
};
  </pre>

  <h3><a name="24.1.4">24.1.4 Oracle Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported basic Qt types and the Oracle database types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Qt Type</th>
      <th>Oracle Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>QString</code></td>
      <td><code>VARCHAR2(512)</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QByteArray</code></td>
      <td><code>BLOB</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QUuid</code></td>
      <td><code>RAW(16)</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>Instances of the <code>QString</code> and <code>QByteArray</code> types
     are stored as a <code>NULL</code> value if their <code>isNull()</code>
     member function returns <code>true</code>.</p>

  <p>The <code>basic</code> sub-profile also provides support
     for mapping <code>QString</code> to the <code>CHAR</code>,
     <code>NCHAR</code>, <code>NVARCHAR</code>, <code>CLOB</code>, and
     <code>NCLOB</code> Oracle types, and for mapping <code>QByteArray</code>
     to the <code>RAW</code> Oracle type. However, these alternative
     mappings have to be explicitly requested using the <code>db&nbsp;type</code>
     pragma (<a href="#14.4.3">Section 14.4.3, "type"</a>), as shown in the
     following example:</p>

  <pre class="cxx">
#pragma db object
class Person
{
  ...

  #pragma db type("CLOB") not_null
  QString firstName_;

  #pragma db type("RAW(16)") null
  QByteArray uuid_;
};
  </pre>

  <h3><a name="24.1.5">24.1.5 SQL Server Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported basic Qt types and the SQL Server database types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Qt Type</th>
      <th>SQL Server Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>QString</code></td>
      <td><code>VARCHAR(512)/VARCHAR(256)</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QByteArray</code></td>
      <td><code>VARBINARY(max)</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QUuid</code></td>
      <td><code>UNIQUEIDENTIFIER</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>Instances of the <code>QString</code> and <code>QByteArray</code> types
     are stored as a <code>NULL</code> value if their <code>isNull()</code>
     member function returns <code>true</code>.</p>

  <p>Note also that the <code>QString</code> type is mapped
     differently depending on whether a member of this type
     is an object id or not. If the member is an object id,
     then for this member <code>QString</code> is mapped
     to the <code>VARCHAR(256)</code> SQL Server type. Otherwise,
     it is mapped to <code>VARCHAR(512)</code>.</p>

  <p>The <code>basic</code> sub-profile also provides support
     for mapping <code>QString</code> to the <code>CHAR</code>,
     <code>NCHAR</code>, <code>NVARCHAR</code>, <code>TEXT</code>, and
     <code>NTEXT</code> SQL Server types, and for mapping
     <code>QByteArray</code> to the <code>BINARY</code> and
     <code>IMAGE</code> SQL Server types. However, these alternative
     mappings have to be explicitly requested using the <code>db&nbsp;type</code>
     pragma (<a href="#14.4.3">Section 14.4.3, "type"</a>), as shown in the
     following example:</p>

  <pre class="cxx">
#pragma db object
class Person
{
  ...

  #pragma db type("NVARCHAR(256)") not_null
  QString firstName_;

  #pragma db type("BINARY(16)") null
  QByteArray uuid_;
};
  </pre>

  <h2><a name="24.2">24.2 Smart Pointers Library</a></h2>

  <p>The <code>smart-ptr</code> sub-profile provides persistence support the
     Qt smart pointers. To enable only this profile, pass
     <code>qt/smart-ptr</code> to the <code>--profile</code> ODB compiler
     option.</p>

  <p>The currently supported smart pointers are
     <code>QSharedPointer</code> and <code>QWeakPointer</code>.
     For more information on using smart pointers as pointers to objects
     and views, refer to <a href="#3.3">Section 3.3, "Object and View
     Pointers"</a> and <a href="#6">Chapter 6, "Relationships"</a>. For
     more information on using smart pointers as pointers to values, refer
     to <a href="#7.3">Section 7.3, "Pointers and <code>NULL</code> Value
     Semantics"</a>. When used as a pointer to a value, only
     <code>QSharedPointer</code> is supported. For example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...

  #pragma db null
  QSharedPointer&lt;QString> middle_name_;
};
  </pre>

  <p>To provide finer grained control over object relationship loading,
     the <code>smart-ptr</code> sub-profile also provides the lazy
     counterparts for the above pointers: <code>QLazySharedPointer</code>
     and <code>QLazyWeakPointer</code>. You will need to include the
     <code>&lt;odb/qt/lazy-ptr.hxx></code> header file to make the lazy
     variants available in your application. For a description of the lazy
     pointer interface and semantics refer to <a href="#6.4">Section 6.4,
     "Lazy Pointers"</a>. The following example shows how we can use these
     smart pointers to establish a relationship between persistent objects.</p>

  <pre class="cxx">
class Employee;

#pragma db object
class Position
{
  ...

  #pragma db inverse(position_)
  QLazyWeakPointer&lt;Employee> employee_;
};

#pragma db object
class Employee
{
  ...

  #pragma db not_null
  QSharedPointer&lt;Position> position_;
};
  </pre>

  <p>Besides providing persistence support for the above smart pointers,
     the <code>smart-ptr</code> sub-profile also changes the default
     pointer (<a href="#3.3">Section 3.3, "Object and View Pointers"</a>)
     to <code>QSharedPointer</code>.  In particular, this means that
     database functions that return dynamically allocated objects and views
     will return them as <code>QSharedPointer</code> pointers. To override
     this behavior, add the <code>--default-pointer</code> option specifying
     the alternative pointer type after the <code>--profile</code> option.</p>

  <h2><a name="24.3">24.3 Containers Library</a></h2>

  <p>The <code>containers</code> sub-profile provides persistence support for
     Qt containers. To enable only this profile, pass
     <code>qt/containers</code> to the <code>--profile</code> ODB compiler
     option.</p>

  <p>The currently supported ordered containers are <code>QVector</code>,
     <code>QList</code>, and <code>QLinkedList</code>. Supported map
     containers are <code>QMap</code>, <code>QMultiMap</code>,
     <code>QHash</code>, and <code>QMultiHash</code>. The supported set
     container is <code>QSet</code>. For more information on using
     containers with ODB, refer to <a href="#5">Chapter 5, "Containers"</a>.
     The following example shows how the <code>QSet</code> container may
     be used within a persistent object.</p>

  <pre class="cxx">
#pragma db object
class Person
{
  ...
  QSet&lt;QString> emails_;
};
  </pre>

  <p>The <code>containers</code> sub-profile also provide a change-tracking
     equivalent for <code>QList</code> (<a href="#24.3.1">Section 24.3.1,
     "Change-Tracking <code>QList</code>"</a>) with support for other Qt
     container equivalents planned for future releases. For general information
     on change-tracking containers refer to <a href="#5.4">Section 5.4,
     "Change-Tracking Containers"</a>.</p>

  <h3><a name="24.3.1">24.3.1 Change-Tracking <code>QList</code></a></h3>

  <p>Class template <code>QOdbList</code>, defined in
     <code>&lt;odb/qt/list.hxx></code>, is a change-tracking
     equivalent for <code>QList</code>. It
     is implemented in terms of <code>QList</code> and is
     implicit-convertible to and implicit-constructible from
     <code>const QList&amp;</code>. In particular, this
     means that we can use <code>QOdbList</code> instance
     anywhere <code>const QList&amp;</code> is
     expected. In addition, <code>QOdbList</code> constant
     iterator (<code>const_iterator</code>) is the same type as
     that of <code>QList</code>.</p>

  <p><code>QOdbList</code> incurs 2-bit per element overhead
     in order to store the change state. It cannot
     be stored unordered in the database (<a href="#14.4.19">Section
     14.4.19 "<code>unordered</code>"</a>) but can be used as an inverse
     side of a relationship (<a href="#6.2">6.2 "Bidirectional
     Relationships"</a>). In this case, no change tracking is performed
     since no state for such a container is stored in the database.</p>

  <p>The number of database operations required to update the state
     of <code>QOdbList</code> corresponds well to the complexity
     of <code>QList</code> functions, except for
     <code>prepend()</code>/<code>push_front()</code>. In particular, adding
     or removing an element from the back of the list (for example,
     with <code>append()</code>/<code>push_back()</code> and
     <code>removeLast()</code>/<code>pop_back()</code>),
     requires only a single database statement execution. In contrast,
     inserting or erasing an element at the beginning or in the middle
     of the list will require a database statement for every element that
     follows it.</p>

  <p><code>QOdbList</code> replicates most of the <code>QList</code>
     interface as defined in both Qt4 and Qt5 and includes support for
     C++11. However, functions and operators that provide direct write
     access to the elements had to be altered or disabled in order to
     support change tracking. Additional functions used to interface with
     <code>QList</code> and to control the change tracking state
     were also added. The following listing summarizes the differences
     between the <code>QOdbList</code> and <code>QList</code>
     interfaces. Any <code>QList</code> function or operator
     not mentioned in this listing has exactly the same signature
     and semantics in <code>QOdbList</code>. Functions and
     operators that were disabled are shown as commented out and
     are followed by functions/operators that replace them.</p>

  <pre class="cxx">
template &lt;typename T>
class QOdbList
{
  ...

  // Element access.
  //

  //T&amp; operator[] (int);
    T&amp; modify (int);

  //T&amp; first();
    T&amp; modifyFirst();

  //T&amp; last();
    T&amp; modifyLast();

  //T&amp; front();
    T&amp; modify_front();

  //T&amp; back();
    T&amp; modify_back();

  // Iterators.
  //
  using const_iterator = typename QList&lt;T>::const_iterator;

  class iterator
  {
    ...

    // Element Access.
    //

    //reference       operator* () const;
      const_reference operator* () const;
      reference       modify () const;

    //pointer       operator-> () const;
      const_pointer operator-> () const;

    //reference       operator[] (difference_type);
      const_reference operator[] (difference_type);
      reference       modify (difference_type) const;

    // Interfacing with QList::iterator.
    //
    typename QList&lt;T>::iterator base () const;
  };

  // Return QList iterators. The begin() functions mark all
  // the elements as modified.
  //
  typename QList&lt;T>::iterator mbegin ();
  typename QList&lt;T>::iterator modifyBegin ();
  typename QList&lt;T>::iterator mend ();
  typename QList&lt;T>::iterator modifyEnd ();

  // Interfacing with QList.
  //
  QOdbList (const QList&lt;T>&amp;);
  QOdbList (QList&lt;T>&amp;&amp;); // C++11 only.

  QOdbList&amp; operator= (const QList&lt;T>&amp;);
  QOdbList&amp; operator= (QList&lt;T>&amp;&amp;);

  operator const QList&lt;T>&amp; () const;
  QList&lt;T>&amp; base ();
  const QList&lt;T>&amp; base () const;

  // Change tracking.
  //
  bool _tracking () const;
  void _start () const;
  void _stop () const;
  void _arm (transaction&amp;) const;
};
  </pre>

  <p>The following example highlights some of the differences between
     the two interfaces. <code>QList</code> versions are commented
     out.</p>

  <pre class="cxx">
#include &lt;QtCore/QList>
#include &lt;odb/qt/list.hxx>

void f (const QList&lt;int>&amp;);

QOdbList&lt;int> l ({1, 2, 3});

f (l); // Ok, implicit conversion.

if (l[1] == 2) // Ok, const access.
  //l[1]++;
  l.modify (1)++;

//l.last () = 4;
l.modifyLast () = 4;

for (auto i (l.begin ()); i != l.end (); ++i)
{
  if (*i != 0) // Ok, const access.
    //*i += 10;
    i.modify () += 10;
}

qSort (l.modifyBegin (), l.modifyEnd ());
  </pre>

  <p>Note also the subtle difference between copy/move construction
     and copy/move assignment of <code>QOdbList</code> instances.
     While copy/move constructor will copy/move both the elements as
     well as their change state, in contrast, assignment is tracked
     as any other change to the vector content.</p>

  <p>The <code>QListIterator</code> and <code>QMutableListIterator</code>
     equivalents are also provided. These are <code>QOdbListIterator</code>
     and <code>QMutableOdbListIterator</code> and are defined in
     <code>&lt;odb/qt/list-iterator.hxx></code> and
     <code>&lt;odb/qt/mutable-list-iterator.hxx></code>, respectively.</p>

  <p><code>QOdbListIterator</code> has exactly the same interface and
     semantics as <code>QListIterator</code>. In fact, we can use
     <code>QListIterator</code> to iterate over a <code>QOdbList</code>
     instance.</p>

  <p><code>QMutableOdbListIterator</code> also has exactly the same
      interface as <code>QMutableListIterator</code>. Note, however,
      that any element that such an iterator passes over with the
      call to <code>next()</code> is marked as modified.</p>

  <h2><a name="24.4">24.4 Date Time Library</a></h2>

  <p>The <code>date-time</code> sub-profile provides persistence support for
     the Qt date-time types. To enable only this profile, pass
     <code>qt/date-time</code> to the <code>--profile</code> ODB compiler
     option.</p>

  <p>The currently supported date-time types are <code>QDate</code>,
     <code>QTime</code>, and <code>QDateTime</code>. The manner in which
     these types are persisted is database system dependent and is
     discussed in the sub-sections that follow. The example below shows how
     <code>QDate</code> may be used within a persistent object.</p>

  <pre class="cxx">
#pragma db object
class Person
{
  ...
  QDate dateOfBirth_;
};
  </pre>

  <p>The single concrete exception that can be thrown by the
     <code>date-time</code> sub-profile implementation is presented below.</p>


  <pre class="cxx">
namespace odb
{
  namespace qt
  {
    namespace date_time
    {
      struct value_out_of_range: odb::qt::exception
      {
        virtual const char*
        what () const throw ();
      };
    }
  }
}
  </pre>

  <p>You will need to include the
     <code>&lt;odb/qt/date-time/exceptions.hxx&gt;</code> header file to
     make this exception available in your application.</p>

  <p>The <code>value_out_of_range</code> exception is thrown if an attempt
     is made to store a date-time value that is out of the target database
     range. The specific conditions under which it is thrown is database
     system dependent and is discussed in more detail in the
     following sub-sections.</p>

  <h3><a name="24.4.1">24.4.1 MySQL Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported Qt date-time types and the MySQL database types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Qt Date Time Type</th>
      <th>MySQL Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>QDate</code></td>
      <td><code>DATE</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QTime</code></td>
      <td><code>TIME</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QDateTime</code></td>
      <td><code>DATETIME</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>Instances of the <code>QDate</code>, <code>QTime</code>, and
     <code>QDateTime</code> types are stored as a <code>NULL</code> value
     if their <code>isNull()</code> member function returns true.</p>

  <p>The <code>date-time</code> sub-profile implementation also provides
     support for mapping <code>QDateTime</code> to the <code>TIMESTAMP</code>
     MySQL type. However, this mapping has to be explicitly requested using
     the <code>db&nbsp;type</code> pragma
     (<a href="#14.4.3">Section 14.4.3, "<code>type</code>"</a>), as shown in
     the following example:</p>

  <pre class="cxx">
#pragma db object
class Person
{
  ...
  #pragma db type("TIMESTAMP") not_null
  QDateTime updated_;
};
  </pre>

  <p>Starting with MySQL version 5.6.4 it is possible to store fractional
     seconds up to microsecond precision in <code>TIME</code>,
     <code>DATETIME</code>, and <code>TIMESTAMP</code> columns. However,
     to enable sub-second precision, the corresponding type with the
     desired precision has to be specified explicitly, as shown in the
     following example:</p>

  <pre class="cxx">
#pragma db object
class Person
{
  ...
  #pragma db type("DATETIME(3)") // Millisecond precision.
  QDateTime updated_;
};
  </pre>

  <p>Alternatively, you can enable sub-second precision on the per-type
     basis, for example:</p>

  <pre class="cxx">
#pragma db value(QDateTime) type("DATETIME(3)")

#pragma db object
class Person
{
  ...
  QDateTime created_; // Millisecond precision.
  QDateTime updated_; // Millisecond precision.
};
  </pre>

  <p>Some valid Qt date-time values cannot be stored in a MySQL database.  An
     attempt to persist a Qt date-time value that is out of the MySQL type
     range will result in the <code>out_of_range</code> exception.  Refer to
     the MySQL documentation for more information on the MySQL data type
     ranges.</p>

  <h3><a name="24.4.2">24.4.2 SQLite Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported Qt date-time types and the SQLite database types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Qt Date Time Type</th>
      <th>SQLite Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>QDate</code></td>
      <td><code>TEXT</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QTime</code></td>
      <td><code>TEXT</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QDateTime</code></td>
      <td><code>TEXT</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>Instances of the <code>QDate</code>, <code>QTime</code>, and
     <code>QDateTime</code> types are stored as a <code>NULL</code> value
     if their <code>isNull()</code> member function returns true.</p>

  <p>The <code>date-time</code> sub-profile implementation also provides
     support for mapping <code>QDate</code> and <code>QDateTime</code> to the
     SQLite <code>INTEGER</code> type, with the integer value representing the
     UNIX time. Similarly, an alternative mapping for <code>QTime</code> to
     the <code>INTEGER</code> type represents a clock time as the number of
     seconds since midnight. These mappings have to be explicitly requested
     using the <code>db&nbsp;type</code> pragma
     (<a href="#14.4.3">Section 14.4.3, "<code>type</code>"</a>), as shown
     in the following example:</p>

  <pre class="cxx">
#pragma db object
class Person
{
  ...
  #pragma db type("INTEGER")
  QDate born_;
};
  </pre>

  <p>Some valid Qt date-time values cannot be stored in an SQLite database.
     An attempt to persist any Qt date-time value representing a negative UNIX
     time (any point in time prior to the 1970-01-01&nbsp;00:00:00 UNIX time
     epoch) as an SQLite <code>INTEGER</code> will result in the
     <code>out_of_range</code> exception.</p>

  <h3><a name="24.4.3">24.4.3 PostgreSQL Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported Qt date-time types and the PostgreSQL database types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Qt Date Time Type</th>
      <th>PostgreSQL Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>QDate</code></td>
      <td><code>DATE</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QTime</code></td>
      <td><code>TIME</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QDateTime</code></td>
      <td><code>TIMESTAMP</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>Instances of the <code>QDate</code>, <code>QTime</code>, and
     <code>QDateTime</code> types are stored as a <code>NULL</code> value
     if their <code>isNull()</code> member function returns true.</p>

  <h3><a name="24.4.4">24.4.4 Oracle Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported Qt date-time types and the Oracle database types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Qt Date Time Type</th>
      <th>Oracle Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>QDate</code></td>
      <td><code>DATE</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QTime</code></td>
      <td><code>INTERVAL DAY(0) TO SECOND(3)</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QDateTime</code></td>
      <td><code>TIMESTAMP(3)</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>Instances of the <code>QDate</code>, <code>QTime</code>, and
     <code>QDateTime</code> types are stored as a <code>NULL</code> value
     if their <code>isNull()</code> member function returns true.</p>

  <p>The <code>date-time</code> sub-profile implementation also provides
     support for mapping <code>QDateTime</code> to the
     <code>DATE</code> Oracle type with fractional seconds that may be
     stored in a <code>QDateTime</code> instance being ignored. This
     alternative mapping has to be explicitly requested using the
     <code>db&nbsp;type</code> pragma (<a href="#14.4.3">Section 14.4.3,
     "<code>type</code>"</a>), as shown in the following example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
  #pragma db type("DATE")
  QDateTime updated_;
};
  </pre>

  <h3><a name="24.4.5">24.4.5 SQL Server Database Type Mapping</a></h3>

  <p>The following table summarizes the default mapping between the currently
     supported Qt date-time types and the SQL Server database types.</p>

  <!-- border="1" is necessary for html2ps -->
  <table id="mapping" border="1">
    <tr>
      <th>Qt Date Time Type</th>
      <th>SQL Server Type</th>
      <th>Default <code>NULL</code> Semantics</th>
    </tr>

    <tr>
      <td><code>QDate</code></td>
      <td><code>DATE</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QTime</code></td>
      <td><code>TIME(3)</code></td>
      <td><code>NULL</code></td>
    </tr>

    <tr>
      <td><code>QDateTime</code></td>
      <td><code>DATETIME2(3)</code></td>
      <td><code>NULL</code></td>
    </tr>
  </table>

  <p>Instances of the <code>QDate</code>, <code>QTime</code>, and
     <code>QDateTime</code> types are stored as a <code>NULL</code> value
     if their <code>isNull()</code> member function returns true.</p>

  <p>Note that the <code>DATE</code>, <code>TIME</code>, and
     <code>DATETIME2</code> types are only available in SQL Server 2008 and
     later. SQL Server 2005 only supports the <code>DATETIME</code> and
     <code>SMALLDATETIME</code> date-time types. The new types are
     also unavailable when connecting to an SQL Server 2008 or
     later with the SQL Server 2005 Native Client ODBC driver.</p>

  <p>The <code>date-time</code> sub-profile implementation provides
     support for mapping <code>QDateTime</code> to the <code>DATETIME</code>
     and <code>SMALLDATETIME</code> types, however, this mapping has to
     be explicitly requested using the <code>db&nbsp;type</code> pragma
     (<a href="#14.4.3">Section 14.4.3, "<code>type</code>"</a>), as
     shown in the following example:</p>

  <pre class="cxx">
#pragma db object
class person
{
  ...
  #pragma db type("DATETIME")
  QDateTime updated_;
};
  </pre>

  </div>
</div>

</body>
</html>
